Support PTransform overriding when input and output types are different

[kennknowles]

#3333 adds ability to dynamically replace PTransforms but does not support replacements when input and/or output types are different. We have to address complexities due to type hints before supporting replacements with different types.

Imported from Jira BEAM-2432. Original Jira may contain additional context.
Reported by: chamikara.