Sorry to disagree - but its dead easy - I updated my Orange Hero to the new .66 ROm on the Orange site, and then found all my paid for Apps were unavailable on the Market. I went back on the the Orange Software download page and downloaded the .5 ROM and connected up the phone, and then clicked on that download. The ".5" version ROM re-installed in about 10 minutes, and Bingo - all my Apps were available again on the Market. I would recommended you backup using My backup - both Apps and Data, before you start.
Makes it easier to restore everything back as you like it.