In the code editor (but it could probably be in others as well), it is very easy to sometimes just click replace and not mean it. So the basic concept is to unreplace (can be any replaced value) or undo (the previous replacements in a stacked order) the last replacement.

Also, sometimes you want to go forwards or backwards, so rather than having a backwards checkbox, just add backwards buttons (saves a lot of switching directions).

Also, sometimes you want to swap the find and replacement text (e.g, changing var1 to var2 and var2 to var1), or do a reverse replacement

Finally, at times you want to find some wildcard strings, and manually replace strings, so it should allow it to find and manually replace each by selecting the found text (so you know what will be replaced) and replacing it with text prompted in a string, that immediately replaces with that value when you type it in

Current buttons
Find next, Replace, Replace Next, Replace All, Cancel

New buttons
Find Prev, Replace Prev, Undo replace
Reverse Replace, Undo replace
Manual Replacement text prompt