Sound like the dreaded update loop. I assume the update is copied onto the hdd and is failing to install and rebooting, then it repeats?? will it let you do anything or is it just looping without giving you any chance to stop it or change the pup?
First of all make sure you have the good versions of the update files for 3.41/3.55. As far as I know both of these firmware updates were released with a HDD upgrading bug and later Sony silently released new fixed versions with same number, look around and make sure they are fixed versions you are trying.
As you were on 2.50+ the recovery menu should be working, make sure you are doing this:
1. Turn off Playstation 3.
2. Hold The power button down; The system will turn on and turn off once again.
3. Once the System has been shutdown, re-press you finger until you hear 2 consecutive beeps
4. When you hear the 2 beeps take finger off power button.
5. You should be promted to plug in your controller via usb and then hit the PS button
6. The Recovery menu should pop up.
If that doesn't work its possible its starting to resume the update proccess before the recovery menu appears, in that case its not such an easy fix, easiest thing I can recommend is getting your hands on a jailbreak dongle and using it (with a psgrade payload) to "unbrick" your console.
Putting in your old HD might allow recovery menu to appear, worth a try, I know it would probably work if you were also to remove ps3 battery for 30 minutes, but that requires stripping down ps3 to the mainboard. Like I said above the easiest fix is probably getting into service mode with a jailbreak dongle.
P.S. I'm wondering how you managed to swap the PUP from 3.41 to 3.55 if you are stuck in a loop, did you get into recovery menu? If you havnt formatted your old drive I suggest trying that to get into recovery menu.
If it is really in an update loop you will not be able to swap the pup. Apparently Sony are releasing a new firmware update in first 2 weeks of this month to hopefully fix this hard drive updating bug that was in the first releases of 3.41 and 3.56 (not 3.55 as I thought earlier)
Until then, if it is the same bug Your only hope is if you havnt formatted your old drive.
Do you get this error - 8002f225? or is there no error code?