Copyright can be enforced for commercial used of non propitiatory property. But as you claimed, you used the s/w for learning purpose only, then it is exempted from copyright or any other IPR LAW. you need not worry, if you have used it for knowledge, research or learning purpose. You write a letter to the concern authority of the Autodesk and let them know that, what was your purpose behind using the previous version and now you do not require it. Even if they did not act or reacted to your letter/email and forced to to buy the licensed copy then you can use this letter/email as an evidence to show your bonafideness in Court.