What is the process for switching back to straight edges after I bevel the edges (and ran out of undo steps)?
You would basically have to remodel it. One tricky way would be, if you double tap G you go into edge slide mode, and then if you hit C you get an extension of the slide and that way you can get the beveled vertices back to where they meet up.
Also there is an addon that has a lot of useful tools to unmake this kind of bevel and other things. It's called Mesh Machine, linking you up:
There are several ways to remove do this,
Edge slide as Omar said is one, but also snapping helps. If you have an object like a cube, you can use vertex snapping to line up the vertices to restore the correct shape, obviously wont work always.
Top Tip: Save often, Save Incrementally.
Did you know you can increase the number of undo steps?
The default is 32, the max is 256.
You can select the edges of the bevel that you want to remove and use dissolve edges. Note: if you're in edge select mode you can use Ctrl+x instead of pressing X and clicking dissolve edges.