I'll offer you some solice from personal experience: I too had issues with previous Serial Numbers from older products. Autodesk is better now than they were several years ago in keeping up with S/N's.
Have you followed the Virtual Agent path?
Also if that does not work I would seek out a member here named philip.s
He is an Autodesker that often helps in these situations.
Regards, Charles Shade
CSHADEDESIGN | AUTOCAD LT | LT-KB | DYNAMIC BLOCKS
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Invalid serial numbers are typically due to the following reason(s):
1. Your product is under active Subscription and has expired the 2009 lease
2. Your product was upgraded under Subscription but has expired
3. Your product was manually upgraded or cross-graded to another product
Even under active Subscription, the 2009 product lease has expired. This means even if you currently have a 2014 LT Subscription, you will need to apply for a previous use serial number key, which most likely will allow you to use the product for 13 months.
If you were ever under Subscription and this auto-upgraded but you never activated a newer version, then you are legally able to run the last registered version. In that case, you will still need to get an authorized activation from Autodesk or your Reseller. Or as Charles has recommended, Philip will be of great assistance.