> 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

-- 
Lukas Renggli
http://www.lukas-renggli.ch