Unzip and copy attachment to the TiVo.
Make sure the files 255 , 255a , setdbversion255.tcl & setdbversion255a.tcl are executable (e.g. chmod +x {filename} )
Run relevant files to change from 255 to 255a or visa-versa.
Now since we are only trying to fool Tivo into there being an OS mismatch, you probably only need to run the setdbversion file, especially for you irrelevant as you play with teletext and hence don't really want to install the teletext disabling pxmpegdecode.o file.
Having said that, it not going to matter if you install the teletext disabling pxmpegdecode.o file because as soon as the tivo reboots, its going to re-install 255 back over what it thought was 2.5.5a and hence repair the pxmpegdecode.o file back to standard anyway!
By the way, one last thing to remember, when Tivo sends an OS update (be it 2.5.5 or 2.5.5a), it contains the original Tivo Kernel (which is not lba48 compatible), so if your HDD is larger than 127Gb, your swapfile will be invalid and your dial-up connections will only work once after a reboot and fail for consequent dials.
I recommend doing these tests on an old 40gb Tivo drive (or similar) to save you having to keep re-installing the Big Disk (vmlinux.px) kernel via the TPIP tool.
Keeping the dream alive.