The Palm desktop software should recognize that you are "upgrading" a device and recognize your user name, which you should use the same user name from the TH55 as on the T5, then the data should migrate over on the first hot sync.
But as a precaution: Check your PC's hard drive for a folder called
C:\program files\palmone\SR (or TH55 user name)\backup
or it may be:
C:\program files\sonyclie\SR (or TH55 user name)\ backup
Regardless - this backup folder is going to contain the programs and there corresponding data for the TH55. Make a copy of this folder - just in case.
Also, the worst case scenario would be that you'd have to beam over the data from Sony to the T5 after it was installed on your PC and then it would be backedup after it's first hot sync.
I took a TH55 and converted to a Treo600 last spring (04) and all I did was as outlined above: Installed the new PalmDesktop and everything came over.
A few programs didn't but Agendus wasn't one of them.
BTW: You can see me over at Iambic aka rsgmoose an Iambic Forums Team Member so if you need further help - just ask.