The first step an update does is copying it to HDD and later it updates NOR and SYSCON , if you are luck it only did the first step. Take the HDD out and format it via PC and cross your fingers that you can go to Recovery Mode and install 3.55 again.
If it already updated too far --> No recovery mode and it will aske for 3.55+ and this means downgarding via flasher must be done. Sorry.
You will loose all data after downgrade, if you have an extra ps3 and you update it you can transfer all your data to another ps3 and then downgrade your ps3 and then transfer data back. Is it a nor ps3 if so some of the flash is stored on the hard drive. If you live in the states I can take care of this for you. I have spare ps3s to transfer data. Thanks
As far as I know the q and a method doesn't work on higher firmwares, cause the keys are needed for it. But it does allow you to install any Cfw on it once the ps3 is downgraded. So I am thinking that it is a flasher downgrade, correct me if I am wrong. I know technodon has talked about q and a before so he might be the one to help. Thanks