How to use Rules to disable access to selected fields while editing an existing node?
You're right, there is no such event like "While editing an existing node". Probably because an event is something that takes like a fraction of a second, while editing may take minutes (hopefully not hours!).

However, you might get it to work by implementing an "edit-in-progress" (= EIP) feature using the Flag module, combined with using the Rules module. Similar to what I described in my answer to "Can the Flag module be used to lock content while editing to prevent concurrent edits?" (BTW, that question is a variation of yours, I don't think it's a duplicate though ...).

After you'd have such EIP-flag available, it should be straight forward to use it in appropriate Rules to "restrict/disable access to certain fields" (eg based on a data comparison to validate selected field values (using their before and after save values).