When I downloaded and installed Windows 7 retail over the copy of Windows 7 OEM (and a Norton trial, and an Office 2007 trial, and various HP media programs) that came with my computer, I went through various methods of trying to get things to work. I tried to use the key that was printed on my sticker, and I tried to use Speccy to read what my OEM copy reported its Product Key as being. No luck, but after some searching I found that Activation Backup & Restore might do the trick. I restored the OEM copy from recovery discs, ran and stored the program in Dropbox, installed the retail copy, ran the software again, and was home free. Unfortunately it doesn't look like there's a version for Windows 8/8.1, but there are apparently other ways of going about it.
Well, I think there's still the command to reset the 30 day trial that works up to three times, so you could extend that to 116-120 days.