As an alternate to doing a copy, just make sure you've committed the form to source control before making the changes. They you just need to do a revert in case of a problem.
A really important thing to understand about designer files is that they aren't magic. They do nothing you couldn't do yourself. In fact, there's nothing you can do in VS that you can't do in Notepad. However, if you used notepad, it wouldn't be much fun (no designers, no declarative programming using property sheets, no visual form building...).

Of course you can break things such that VS can't open them in a designer, but since VS often breaks things so they can't be opened in a designer, you'll likely need to start getting comfortable opening the designer files anyway!
I also second not using Replace All. Every time I've done that, I ended up breaking something, either because there was some match that wasn't appropriate or because I intended to replace all in the current file, but had entire solution selected.