Alex,
Also, one other option exists.? The Yahoo download is in MBOX format.? There are several MBOX reader, viewers and robust editors on the web.?
Depending on how your YG was setup over there, the MBOX could have spam, Yahoo & AOL ads and even pron postings to adult websites.? Thus, the editor is needed before migration those messages over here.? Out of my 4 groups, 2 have the wild side junk in them from the mentioned events.?
Again, there are options.? Just figure out the best path that meets your groups needs getting over to GroupsIO.? You are in the driver's seat as the transfer window from YG to GIO is now past.
BMaverick