If you don't know what was originally installed that could be an issue. My COA sticker just says Windows 8. I would think if the PC came with 8 Pro the sticker would say Pro. If it just says 8 I'd assume its 8 Core. I haven't seen an 8.1 COA sticker.
Getting install media is an issue if all you have is an OEM key. The only official download, other than MSDN, will not accept OEM keys. That download is a custom Core Pro download depending on what key you use. If you enter a Pro key you get Pro and it will not accept a Core key for install.
My MSDN ISO's are multi edition, Core and Pro. The key you enter during install determines what's installed. If an OEM key is detected the matching version is installed automatically. As far as I know my MSDN ISO's are identical to the retail DVD's. There is no ei.cfg file to delete on Windows 8 ISO's. There is a trick to bypass having to enter a key though. The end result is you install with a generic key and have 30 days to activate. You'd still have to know which version Core or Pro to install. It's likely in the tutorials somewhere.
The generic keys are blocked for this download,
Solved Activate retail Windows 8.1 with Windows 8 Product key. , its retail keys only. The reason I posted it was just to let you know that, even if your PC came with Windows 8.0, you can still clean install 8.1 and activate with your 8.0 key. That's all.
I personally like the new activation setup. I have access to MSDN though, so ISO's are not an issue for me.
With Windows 7 and earlier, each OEM had a master product key and all their PC's activated with that key. The custom OEM install media had the key on the disk. The COA sticker was their so you could use regular install media. The product code on the sticker was never used for activation. You actually had to do a phone activation if you tried to use it. at least the first time anyway.
Now each Windows 8 PC that has an OEM install has a unique Product key embedded in the BIOS. Matching that key to the what is printed on the sticker would just complicate things. If the machine that prints the stickers gets out of sequence things get really messed up. I can see why they didn't print the keys on the stickers for OEM PC's. If the keys weren't in the BIOS they would have to be entered manually for each PC, not something the OEM's are going to want to do. It's got to be automated somehow.