In my case, missing my filters (which merely apply a correct label, but still keep the item in the Inbox) would not have caused me any big problem. But some people do much more aggressive filtering, relegating unrecognized items to some label/folder to be looked at "eventually".
As for updating my contacts list to change all "@
muug.mb.ca" to "@
muug.ca", that is mainly so that new posts to RoundTable, etc., go to the new addresses. Replies (to RoundTable, etc.) would of course go to the new addresses anyway, if MUUG has adjusted the reply-to addresses.