REF: simplify Block.replace
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I guess this technically a bug fix as previously replace on a Period would incorrectly cast, pls add a whatsnew, merge on green (or ping if you want).