Is it possible to add and correct fields for past events?

Hi, we just set up our first Universal Forwarder which now works as expected. But it didn't do so initially, before we had all set up correctly. We now have the problem, that the first events we forwarded didn't have the fields we defined for that source type. I later added a props.conf for the forwarder, so the new events have the correct fields. I also had one copy & paste error on the first try, so the second bunch of events has one wrongly spelled field name. Is there a way to correct these mistakes? Or should we just delete all old events and re-upload them manually? Thanks a lot!