Why do you not have that flexi on a disc? You had to install it on there somehow, I know there are a few for sale on ebay and they are cheap if you get no help from flexi
I don't think the install media is the issue, it's deactivating an existing install that is. It would be like having CS5 on a computer that crashed before you could deactivate it to put it on another computer (providing, it's already on 2 computers). Doesn't matter if one has the disc or not.
Speaking of discs and I know this is beyond the original question, but I thought I would mention it.
I would start creating backups of those install discs (only the ones that one would want to have a backup of) as ISO images. Most any disc burning software would be able to do this. I wouldn't suggest compression software even though most can create ISO files. The reason why is that compression software will truncate certain file names without warning and truncated names on an install disc is no bueno. It won't work at that point. Disc burning software do give you a warning and you can tell it to keep the original file name as well.
Most OSs as well, will recognize ISOs as virtual discs and mount them just like they would if it was in a physical disc drive.
There are 3 reasons why I would suggest doing this, but the biggest one is that based on what family members have bought very very recently, it seems like the optical drive is on it's way out, (going the way of the floppy drive I reckon) in this internet/download centric computing environment that we are in, combined with cheap(er) HD storage.
Just a suggestion.