Since the ps3 with the new trophys cannot be sync'd via psn (as you do not want to upgrade it) your best option to transfer trophys is this:
1) On the 3.55 CFW ps3 you want to update, create a user profile with the same name as the one with the trophies.
2)Skip this step if you only have one profile on the update ps3. Go into your favorite file manager (I prefer multiman's) and navigate to /dev_hdd0/home/ you will see multiple folders similar to /00000001/ These are the profiles stored on the ps3. Make note of the one that has a creation date of today, as it is the profile you just created. If you only see one, you only had one profile.
3)Go to the ps3 with the trophies. Open a file manager and browse to /dev_hdd0/home/ again you may see multiple folders indicating multiple profiles.
WARNING IT MIGHT NOT HAVE THE SAME NUMBER AS THE OTHER PS3!!!
To tell the difference you need to know the date you last saved a game playing with that profile. Open one of the numbered folders, go to savedata, and look for the recent game save. If it is not there try the next numbered profile folder.
4) At this point you may have noticed there is a folder in there called trophy. Either copy the folder to an external device (thumbdrive) or ftp it over to another device.
5)Take the thumbdrive and stick it in the ps3 to be upgraded. Navigate to /dev_hdd0/home/(the number I said to save)/trophy/ and copy all of the contents into it.
6)After the transfer is complete, turn off the upgrade ps3. Go into recovery menu by holding the power button till it power cycles, then hold it again til you hear two quick beeps.
7) Select rebuild database and wait for it to complete.