I can't find a way to change the next command at runtime when debugging in PyCharm (see: http://pymotw.com/2/pdb/#jump-back),
This has been around in VS/PyTools for ages ("Set next statement"). Some people on JetBrains' forum said it was not possible but I'm not sure if they can be trusted... (https://youtrack.jetbrains.com/issue/PY-9537)
Do anyone has more information on the topic?
You currently can't. Here is the feature request, please upvote it - https://youtrack.jetbrains.com/issue/PY-14306
As far as "undoing" by going back lines to match a previous state, yes that is much harder. Just re-running the previous line shouldn't be too hard (understanding that it's not going to undelete or undo state changes)