As always, it's important to note that these PS3 Debug / TEST updates will currently NOT install on a retail PS3, and so it is intended for examination and comparison purposes only unless you own a Dev unit.
For those curious, running this updater on a retail PS3 will not damage it, however, it will give the following error before the installation completes: The data type is not supported. (8002F029)
Stay tuned for more PS3 Hacks and PS3 CFW news, follow us on Twitter and be sure to drop by the PS3 Hacks and PS3 Custom Firmware Forums for the latest PlayStation 3 scene updates and homebrew releases!
what model of debug you have, an old dechc-a/j or a new slim test? i think its a new slim, so my idea doesn't work for you, i had an old phat ps3 test, a dechca, i always had the possibility to downgrad to 1.0 and any other version. do you have downloaded the 2.5 update from this site, try this one.
hope it helps you, would be sad to here that you can't downgrade anymore.