Here's the run-down of how to remarry WITHOUT losing your BD movie playback (I've done this twice this week...)
You need to downgrade to either 3.15 (16Mb NOR) or 2.30 (256Mb NAND) firmware - I do this by downgrading in steps, eg, 3.55 to 3.41, then 3.41 to 3.21, 3.21 down to 3.15 or 2.30 etc. I get no errors from downgrading this way and believe me, I've spent HOURS studying the outcomes.
Once you're on 3.15 or 2.30 you need the dev_usb000 folder with the BDROM marry files. Inside the file archive is a folder called 'standalone' containing a config file. You need to scroll down to the bottom and change the '0' to '1' next to the DriveInit option & also Update system software - then save changes.
Copy all those files + either the 3.15 or 2.30 firmware onto your FRESHLY FORMATTED Usb drive & stick it in the far right usb port (your PS3 should be in FSM before this point)
You should see a black screen with txt -
The top option "Drive Init..." should say "PASS" after 2-3 mins
The second option will install the 2.30 or 3.15 PUP you have on the USB drive (this is needed!!) Give it a few minutes...
Your PS3 should turn off when complete.
The next step is VERY IMPORTANT!!!
Remove your USB drive (obviously)
Turn your PS3 on, you should still be on 2.30/3.15 and be in FSM
Stick a Bluray film in your drive and play it for a few minutes (until it reaches the top menu)
Eject the disc, then power off your console.
Leave your PS3 in FSM and then upgrade your firmware in steps again.
Using the LV2Diag file, start with the 3.15 (if you're on 2.30) then jump to 3.21, then to 3.41, then to 3.55 OFW. After each firmware update, put your Bluray film in the drive and check you still have playback.
At this point you need to EXIT SERVICE MODE. Once you've done that, power up again and copy the 4.00 OFW PUP into a folder on your FRESHLY FORMATTED USB drive (PS3/UPDATE/PS3UPDAT.PUP) and update through the system menu as normal.