if I have some Seaside 2.3 and upgrade to 2.8 (somehow) will it break my old applications?
Detailled differences are listed on the web-site. See "What's new in Seaside 2.7", "What's new in Seaside 2.8" and "What's new in Seaside 2.9":
http://www.seaside.st/community/development
Seaside 2.3 is very old, it is likely that you need to refactor big parts. Especially the rendering engine has changed.
Cheers, Lukas