I had xXx rom installed on my OnePlus 7 not pro version before I bought my OnePlus 7 pro. I am not responsible for any rats that wake from the dead as Zombie Rats and eat your phone.
1)Download the stock Oxygen OS rom to the sd card on your phone from the Oxygen OS website, or from the Oxygen Updater app, or from XDA. Just make sure it is the Stock Rom though. (If you Do Not have a Custom Rom, Skip to Step 3.)
2) Go into TWRP and flash your Custom Rom's Uninstaller, if there was one, that the developer made with the custom rom. (do not reboot!!!!)
3) In TWRP, flash the Magisk uninstaller. (do not reboot!!!!!)
4) In TWRP, flash the Oxygen OS Stock Rom.
5) Buy the OnePlus 7 Pro like you know you should have in the first place.
The Custom Rom, Magisk, and TWRP will No Longer be on your phone.
If you want to Relock the bootloader, follow the guide in the link below. I needed to relock my bootloader in order to return my OnePlus 7 Not pro and this is the guide that helped me to do it. It says OnePlus 7 Pro in the guide, but it also worked for me on my OnePlus 7 not pro.
Mod edit: Link removed!