Without a disk or installer you have saved, there is nothing you can do to move the program from an old computer to a new computer to use it. Sorry. Autodesk does not store old software, they expect you to store your own downloads since you paid so much money for it all (if it was a stack of dollar bills you'd know where it is safely stored, right? same for expensive software downloads and disks).
Search the web for disks or downloads from 3rd party sites that will expect you to pay $$ for the privilege, just be careful, lots of scammers out there.
Note: your AutoCAD2015 is not fully designed for in Win10 http://autode.sk/1IY4C2X
Your 2015 software is also designed to stop reauthorizing as of March 2021, which may get triggered by a Win10 update, so have a backup plan in place that is going to cost you $$ to fix https://knowledge.autodesk.com/support/autocad/learn-explore/caas/simplecontent/content/previous-ver...
As a general rule, old software needs to be best maintained on old computers/operating systems: If you still have your old Win7 PC and all works well, hang on to it for as long as it will last, get a new computer for whatever else you need as a separate device.
HTH
