Wikipedia:Restoring part of a reverted edit
Restoring part of a reverted edit is a recommended practice in online collaborative writing.
Often when an article version contains more than one disagreeable passage, it is easy to revert to a previous version. This gets rid of all the "mistakes" in a few seconds, but it also can eliminate "good stuff", discourage other editors, and spark an edit war.
Don't throw out the baby with the bath water
If some of an edit is bad, but the other half is good, please do not revert all of it. It's better to mine the old version for good stuff and selectively keep it. Just throw out the bad stuff.
Save my baby!
If someone discarded some good stuff when reverting, please don't revert the reversion. This can spark an edit war.
Just find some of the good stuff and put it into the current version. Then do a diff and look for more good stuff. Repeat as needed.