I was reading about the new update and i recall talk about the license working only twice so you could put it both on “your” mac and “your” windows at the same time. what if one of them has a virus and you need it wiped clean how would you be able to get it back on that computer?
By emailing support.
no steps needed.
you can have 2 machines active so if you have to wipe and setup the same os etc again it will be the same machine 
Bingo. If it’s the same OS and the same hardware you can reinstall without hassle. If you change hardware by moving from an old machine to a new one, or by replacing significant components on the current machine (new motherboard or hard drive for example) then you can do as Matt said and contact support (support@unity3d.com) and we’ll take care of you.