I've never done an upgrade to my blog engine because I think that I've not
advantages to migrate from my actual customized .Text engine to
CommunityServer. My actual blog engine works really good with
my customizations (no spam for example) and my eyes love the actual interface
so, can you tell me a good reason to migrate?
Today I've played a bit with the beta version of CommunityServer
1.2 (not publicly available yet but ready to test with a live demo provided
by Jayson Knight) and I've to admit that my opinions could change now... 
CS 1.2 has a nice new user interface and a good new administration
panel, more clear than the previous. If you're interested on CS, I'll recommend
to play with this live demo a bit (login with Bob as username and
iambob as password).
When CS 1.2 will be officially out, I think I could plan the famous
migration... Telligent is doing a good work!