I'm willing to buy Windows 8 Pro (upgrade), but...

Hello guys, so I'm really willing to buy Windows 8 Pro upgrade, but I have this damn doubt.

So, I format my PC every once in a while and I have Windows 7 Ultimate DVD, and, of course, everytime I need I just install it and it's okay. But, for Windows 8, as it is only an upgrade, I'm wondering if I'll be able to install Windows 7 and then upgrade it to Windows 8 as many time as I want, once I've bought the upgrade... I really need to know, because I format my PC a lot of times in a year and if it is limited I won't buy it.

Thanks in advance,