Not so fast. I just had this dilemma last night. I upgraded to Lion on my iMac. Launched Logic Pro 8, got the error message that it didn't work. An Apple KnowledgeBase Article suggested trashing Logic 8 from the Applications Folder and reinstalling. Way wrong! The installer that came with Logic Pro 8 is PowerPC Based (runs on Intel in Rosetta), which Lion no longer supports, SO YOU CAN'T REINSTALL LOGIC PRO 8 IN LION. I called local apple store, their genius folks confirmed this, and told me that the upgrade to Logic Pro 9 would not install without a current copy of Logic 8 on the drive.
I was lucky, though. I went into Time Machine, and then restored my apps folder to the previous backup with Logic Pro 8 in it, bought the upgrade, and installed it. When I installed it, it did look for "items on my computer" to determine whether I was eligible for the upgrade install. Finding it, it installed. The Apple Store guy suggested calling Apple, explain your situation, and ask them for an upgrade code that will let you upgrade without the full version CD, or give you the full version CD withough the price.
Here's my dilemma now. What if my HD crashes? Apparently, I can't just install Lion, or I'll be in the same position as the OP. Same thing if I buy a new Mac, which I intend to do in fairly short order. At that point, I won't be able to install Logic Pro 8, then upgrade to 9. I am not sure what the answer is, but Apple needs to fix this hole.