Cacus:
I did the same thing nearly 2 years ago - an M505 to a T3. Big jump for me to OS5 at the time, and I worried certain apps would not work.
All my apps worked - even older (and some way older!) OS3/OS4 types like TomCatalog and Always On.
Hacks are relatively easy to handle:
TealMaster will handle, as genoahaus said, certain older OS4 hacks safely, and put a red "!" next to ones that won't work before you launch them. OS5 Hacks WILL NOT work in TealMaster - but they WILL WORK with YAHM.
http://yahm.palmoid.com/
So I simply have the OS5 hacks running under YAHM ("Yet another hack manager," freeware). Hacks "checked on" under YAHM should be checked "off" under TealMaster. Hacks on under TM should be "off" under YAHM.
I run TealLaunch under T/Master, "No Dot" and another one I can't remember right now under YAHM - no problems. YOu can have 2 hack managers running independently and I haven't had a crash.
As far as non-hack stuff, I would install them one (or two) at a time - not just "en masse" so that you can check for problems - but there shouldn't be any.
I personally moved all of the apps in my backup folder to a temp "hold" folder, hotsynced only the addresses/calendar stuff - and then installed apps slowly. Under the HotSync Manager "custom" tab in the T3 Desktop version, you'll see listings for the "old" and "new" conduits listed. The NEW NAMES for T3 PIMS are "contacts, memos, tasks and calendar."
After you've synced your contacts/dates/memos go into hotsync "custom" and uncheck the names for the OLD CONDUITS for the 515 (addresses, to-dos, memo pad and datebook) and leave CHECKED AND ON the NEW "PIMS" - contacts, memos, tasks and calendar." The T3 uses different names on the Palm desktop to synch information, and I don't like any conflicts between "old and new." The ones you'll have on, to reiterate, are:
contacts
tasks
calendar
memos
Set these to your preference (synch, handheld overwrites, desktop overwrites). Set the others to "do nothing." This should alleviate some problems people have. I think in the beginning you need them on so Palm Desktop can "gather the info" from the old 515 PIMS to transfer - I'm not sure.
Good luck with the upgrade - you'll LOVE THE T3 and shouldn't have much of an "app upgrade" problem as I said earlier. Very few "won't work," and you'll know soon enough if you have one of those rare ones. I was amamzed how many older apps DID WORK just fine!
Cheers.