I am trying to activate AutoCAD LT 2013 from 2008. When it asks me for my previous serial number, I get this message:
The serial number you entered for your previous product is not valid. Try again. (18)
Jonathan Carlos in Customer support responded saying that my old serial number has already been upgraded to another new serial number for AutoCAD 2013 LT, which doesn't match the one I have.
What do I do now?
You need to ask Autodesk or your Reseller what your latest serial number was before the upgrade. It may have been upgraded under previous Subscription without your knowledge. They'll have the number.
If you cannot obtain it, send me your 2008 serial number in a PM and I will see if I can look it up for you.
Confirming that the previous version serial number is attached to an active subscription contract, and has consequently been upgraded multiple times to the current 2013 version.
With that being said, I would contact the Contract Manager on the account (C. Ryall) to inquire about receiving the relevant licensing info, at which point you would potentially be able to return the recently purchased upgrade license.