The new redesign will involve maybe 20 minutes of downtime while the actual migration takes place, and I'll give ample warning ahead of time when that is about to happen. It will probably take about a week or two for me to make all the necessary template changes on a test server first, so the upgrade may not happen until mid April or thereabouts.
Re: avatars, you will still be able to upload your own avatars as always. The gallery is just a way for folks who are less tech-savvy to be able to browse some options and select one. (Avatars you upload yourself will not be added to the gallery for others to choose.)