I moved across with two large groups.  No rejoining is necessary.  For the average memeber the transfer is seamless.

I am also on other groups that have made the switch and as far as I can remember all the ordinary members saw was a single message saying that they were now a member of the "new" group, with an option to quit if you disagreed with this change.

-- 
Brian Rumary
England
brian(at)rumary.co.uk