Could convert() also downcast by default?

[lawremi]

Having convert() support automatic downcasting could be an alternative to #420 for enabling constructing an object based on an instance of a parent class. I am thinking it would behave like this:

args <- props(from)
if (".data" %in% names(formals(to)))
      args$.data <- S7_data(from)
do.call(to, args[names(formals(to))], ...)

Note that the ... would allow specifying non-optional subclass properties at construction time.