How to switch OG group members in bulk to another group
OK. Finally got it right. The easiest way to do this is to do a dump of the og_membership table into a text file.

Then run a "search and replace" for the gid, replacing the old gid with the new one. A simple word processor is ideal for this. Any string replacement can be handled this way.

Then importing that text file into the og_membership table.

Voila! All members have switched to the new group and they don't even know it! :)