diff --git a/spec/namespaces/indices.yaml b/spec/namespaces/indices.yaml
index b2b0e64de..442021c5b 100644
--- a/spec/namespaces/indices.yaml
+++ b/spec/namespaces/indices.yaml
@@ -842,6 +842,7 @@ paths:
       operationId: indices.get_upgrade.0
       x-operation-group: indices.get_upgrade
       x-version-added: '1.0'
+      x-version-deprecated: '1.0'
       description: The `_upgrade` API is no longer useful and will be removed.
       externalDocs:
         url: https://opensearch.org/docs/latest
@@ -856,6 +857,7 @@ paths:
       operationId: indices.upgrade.0
       x-operation-group: indices.upgrade
       x-version-added: '1.0'
+      x-version-deprecated: '1.0'
       description: The `_upgrade` API is no longer useful and will be removed.
       externalDocs:
         url: https://opensearch.org/docs/latest
@@ -865,6 +867,8 @@ paths:
         - $ref: '#/components/parameters/indices.upgrade::query.ignore_unavailable'
         - $ref: '#/components/parameters/indices.upgrade::query.only_ancient_segments'
         - $ref: '#/components/parameters/indices.upgrade::query.wait_for_completion'
+      requestBody:
+        $ref: '#/components/requestBodies/indices.upgrade'
       responses:
         '200':
           $ref: '#/components/responses/indices.upgrade@200'
@@ -1825,6 +1829,7 @@ paths:
       operationId: indices.get_upgrade.1
       x-operation-group: indices.get_upgrade
       x-version-added: '1.0'
+      x-version-deprecated: '1.0'
       description: The `_upgrade` API is no longer useful and will be removed.
       externalDocs:
         url: https://opensearch.org/docs/latest
@@ -1840,6 +1845,7 @@ paths:
       operationId: indices.upgrade.1
       x-operation-group: indices.upgrade
       x-version-added: '1.0'
+      x-version-deprecated: '1.0'
       description: The `_upgrade` API is no longer useful and will be removed.
       externalDocs:
         url: https://opensearch.org/docs/latest
@@ -1850,6 +1856,8 @@ paths:
         - $ref: '#/components/parameters/indices.upgrade::query.ignore_unavailable'
         - $ref: '#/components/parameters/indices.upgrade::query.only_ancient_segments'
         - $ref: '#/components/parameters/indices.upgrade::query.wait_for_completion'
+      requestBody:
+        $ref: '#/components/requestBodies/indices.upgrade'
       responses:
         '200':
           $ref: '#/components/responses/indices.upgrade@200'
@@ -2263,6 +2271,11 @@ components:
                   $ref: '../schemas/indices.update_aliases.yaml#/components/schemas/Action'
             description: The definition of `actions` to perform
       required: true
+    indices.upgrade:
+      content:
+        application/json:
+          schema:
+            $ref: '../schemas/indices._common.yaml#/components/schemas/UpgradeRequest'
     indices.validate_query:
       content:
         application/json:
@@ -2528,7 +2541,11 @@ components:
             type: object
             additionalProperties:
               $ref: '../schemas/indices._common.yaml#/components/schemas/TemplateMapping'
-    indices.get_upgrade@200: {}
+    indices.get_upgrade@200:
+      content:
+        application/json:
+          schema:
+            $ref: '../schemas/indices._common.yaml#/components/schemas/IndexGetUpgradeStatus'
     indices.open@200:
       content:
         application/json:
@@ -2738,7 +2755,11 @@ components:
         application/json:
           schema:
             $ref: '../schemas/_common.yaml#/components/schemas/AcknowledgedResponseBase'
-    indices.upgrade@200: {}
+    indices.upgrade@200:
+      content:
+        application/json:
+          schema:
+            $ref: '../schemas/indices._common.yaml#/components/schemas/IndexPostUpgradeStatus'
     indices.validate_query@200:
       content:
         application/json:
diff --git a/spec/schemas/indices._common.yaml b/spec/schemas/indices._common.yaml
index 2f08e5827..316cba4e3 100644
--- a/spec/schemas/indices._common.yaml
+++ b/spec/schemas/indices._common.yaml
@@ -1144,6 +1144,60 @@ components:
         - mappings
         - order
         - settings
+    IndexGetUpgradeStatus:
+      type: object
+      properties:
+        size_in_bytes:
+          $ref: '_common.yaml#/components/schemas/ByteCount'
+        size_to_upgrade_in_bytes:
+          $ref: '_common.yaml#/components/schemas/ByteCount'
+        size_to_upgrade_ancient_in_bytes:
+          $ref: '_common.yaml#/components/schemas/ByteCount'
+        indices:
+          type: object
+          additionalProperties:
+            $ref: '#/components/schemas/UpgradeStatus'
+    UpgradeStatus:
+      type: object
+      properties:
+        size_in_bytes:
+          $ref: '_common.yaml#/components/schemas/ByteCount'
+        size_to_upgrade_in_bytes:
+          $ref: '_common.yaml#/components/schemas/ByteCount'
+        size_to_upgrade_ancient_in_bytes:
+          $ref: '_common.yaml#/components/schemas/ByteCount'
+    IndexPostUpgradeStatus:
+      type: object
+      properties:
+        _shards:
+          $ref: '_common.yaml#/components/schemas/ShardStatistics'
+        upgraded_indices:
+          type: object
+          additionalProperties:
+            $ref: '#/components/schemas/UpgradeVersionStatus'
+    UpgradeVersionStatus:
+      type: object
+      properties:
+        upgrade_version:
+          $ref: '_common.yaml#/components/schemas/VersionString'
+        oldest_lucene_segment_version:
+          $ref: '_common.yaml#/components/schemas/VersionString'
+    UpgradeRequest:
+      type: object
+      properties:
+        allow_no_indices:
+          type: boolean
+        expand_wildcards:
+          type: string
+          enum:
+            - all
+            - closed
+            - none
+            - open
+        ignore_unavailable:
+          type: boolean
+        wait_for_completion:
+          type: boolean
     IndexError:
       type: object
       properties:
diff --git a/tests/default/indices/upgrade.yaml b/tests/default/indices/upgrade.yaml
new file mode 100644
index 000000000..281f94029
--- /dev/null
+++ b/tests/default/indices/upgrade.yaml
@@ -0,0 +1,57 @@
+$schema: ../../../json_schemas/test_story.schema.yaml
+
+description: Test upgrading index using both GET and POST methods.
+epilogues:
+  - path: /movies
+    method: DELETE
+    status: [200, 404]
+prologues:
+  - path: /_bulk
+    method: POST
+    parameters:
+      refresh: true
+    request:
+      content_type: application/x-ndjson
+      payload:
+        - {create: {_index: movies, _id: movie1}}
+        - {director: Bennett Miller, title: The Cruise, year: 1998}
+        - {create: {_index: movies, _id: movie2}}
+        - {director: Nicolas Winding Refn, title: Drive, year: 1960}
+chapters:
+  - synopsis: Trigger index upgrade (POST).
+    path: /{index}/_upgrade
+    method: POST
+    parameters:
+      index: [movies]
+    request:
+      payload:
+        allow_no_indices: true
+        expand_wildcards: open
+        ignore_unavailable: true
+    response:
+      status: 200
+
+  - synopsis: Check index upgrade status (GET).
+    path: /{index}/_upgrade
+    method: GET
+    parameters:
+      index: [movies]
+    response:
+      status: 200
+
+  - synopsis: Trigger upgrade (POST).
+    path: /_upgrade
+    method: POST
+    request:
+      payload:
+        allow_no_indices: false
+        expand_wildcards: closed
+        ignore_unavailable: false
+    response:
+      status: 200
+
+  - synopsis: Check upgrade status (GET).
+    path: /_upgrade
+    method: GET
+    response:
+      status: 200
\ No newline at end of file