I think what Chrome meant was that if you want to DL the DLC and not pay again you need to deactivate the other machine from that PSN ID.
I did all of the deactivation and activation steps and downloaded everything successfully during lunch today.
New system is up and running with ALL of my old GT5 data so I'm happy about that.
Of course, it sucks that I don't have the hundreds of replays and pictures, but the best pictures were exported into Flikr and hard drives so that's fine.
Replays WOULD have been possible to keep if I hadn't wiped the previous machine as one of my attempts to fix it. Oh well. (but even then, doing an export to XMB on each fo them would have taken too long for it to be worth it.)
I still have the memories
THANKS TO EVERYONE FOR HELPING