If you have a valid Product Key for W7 you would be able to re-install W7.
If you don't have the W7 install disk you may be able to borrow one or have one copied. You have to make certain that the install disk is the same variant (of W7) as the key.
I've copied Windows disks without any trouble....it's the Product Key that is the piracy control.

Edit:
If you want to save W8 you can set your machine up to "Dual Boot".
Partition your hard drive in half (or similar)
On startup the machine will default W8 and you have 30 seconds to choose W7. (typically, without tweaking, the default OS is the most current)

The above is a good way to get used to a new OS without dumping anything.
My 83 year old mom used an XP/Vista dual boot machine.