Yes, I think that is true...
It's like this, I guess...
If you buy a software, you get to have several chances to upgrade with reasonable price, but if you do not get that chance on time, you don't get to get the upgrade at all...
I do not know why you want to upgrade to Flash 5 at this moment, but I think your best bet is to find somebody who's willing to sell a copy of Flash 5 for you...
I just googled real quick and found somebody selling Flash 3 for Mac, CD at $99... Do some searching...