If from is an instance of to, i.e., identical(S7_class(from), to) is TRUE (assuming from is an S7 object), then no dispatch should occur.

This is not (just) about efficiency. If there are convert() methods for downcasting to to, one of them will be called even if there is no need to downcast, potentially resulting in data being overwritten, or other weird things happening.

I'll also note that methods::as() is idempotent when to is class_any, but class_any is not allowed with convert(). Somewhat pedantic, but I thought it was worth mentioning.