Have you already tried this?
GarageBand: Re-downloading Learn to Play Artist Lessons
- Log in to your Apple Store account and vIew your order history.
- Click any lesson to download it again.
For me this worked beautifully. After signing into the account and viewing the Order history, I found Sting waiting for me to download again.
Added: I just reread your question, and I think I missed your point, sorry.
Your purchases done before 18 month probably will not be available this way. Try the telephone number at the top of the page and contact the Store; at least to claim a refund for the last two purchases and ask them to to assist with restoring the previous purchases.
Do you have a backup of your old installation? The lessons should be installed in the System Library:
/Library/Application Support/GarageBand/Learn to Play/
If your TimeMachine backup goess back in time long enough to access the time before your upgrade, you simply can restore the contents of the "Learn to Play" folder.