BUG: Fixed Series.replace for EA with casting
Closes https://github.com/pandas-dev/pandas/issues/34530
@TomAugspurger 

in the OP

> The code does not work on Pandas Version 1.0.4. It throws an exception. However, it works on Pandas Version 1.0.1, and also with dtype="int64.

is this PR a candidate for backport? Yeah, I suppose so. Let me update the whatsnew. OK should be ready once CI passes @simonjayhawkins. I've added to the 1.0.5 milestone. thanks @TomAugspurger  @MeeseeksDev backport to 1.0.x

