From what i've been reading lately it may be a good thing u ain't a beta tester and updated your slim yet or glad u have a 0225 fw,as with the new beta dash update,and soon to be released dash it seems that if u do update the dash on your newly flashed 9504,it will be reflashed by the new dash so no older backups will run only ap25 backups,and u'd still need to reflash your 360 again to get these running or the new adg3 discs, and you'll probably be flagged so until the 360 mod gods find a way around the new dash update it may be better to hold off until something is sorted if u still want to play all backups (not online as i put) just stay at 12625 dash.
It seems like only fat Hitachi drives seem the only ones to be able to avoid this naughty situation.
No worries though c4eva and others are already on the case and sorting it.
If u got the cash and desperately want to play 360 backups then buy a lizard if your a complete noob, or a 360 pro if you know a little more and then u can flash from a laptop.
Theres also a molex external adapter going to be released that is run by 4 aa batterys so with a lizard u won't even need a pc/laptop.
My advice is wait until the new dash is released then we'll see exactly what is going to happen with the 360 scene and also with the upcoming release of the xkey and xode,life might be a bit easier again for every 360 owner.