The good news is that you can.
The bad news is that it's made as difficult as possible, Courtesy of Microsoft through their retailers.
I decided to replace my 6-yr-old Pentium 4 32-bit desktop late in 2013.
I chose an I5 Lenovo, which came with Win 8 installed. Win7 was not a free option (as it has been in the past, when buying a new system, to downgrade to the previous stable Windows version.)
I could have it done if I bought a new Win 7 64-bit software, $130, and pay for the "tricky installation," so described, an additional $150 or so. I opted not to.
I truly hope you can find a more rational option, perhaps preserving the Win 8 64-bit installation already paid for, with the option of re-installing it at a later date.
I never figured out how to do that without the onerous extra expense. I will be looking at this thread for hope of a reasonable alternative.
I chose an I5 Lenovo, which came with Win 8 installed. Win7 was not a free option (as it has been in the past, when buying a new system, to downgrade to the previous stable Windows version.)
You can "downgrade" Win 8 Pro to Win 7 Pro at no cost. Home versions, no.