i think "Upgrade" (overwrite those old files with 3.4 new files, change config.asp then run setup.asp !) can keep your old posts/members data. but it's NO way to keep the installed Mods code modification anymore...
you'll need to reinstall those Mods, or do the Mods code modified to your new 3.4 original code again. (some Mods will still work on 3.4, but most of them need a updated version for 3.4 new code.)