I suggest checking if FL Studio or the manufacturer has any information regarding what version it would need to run third-party plug-ins like Ampire. If I had to hazard a guess, though, I'd say you'd probably want to update your computer. (Mainstream support for Windows 8.1 ended in 2018 and extended support will end in 2023 so it's probably a good time to go ahead and update anyway.) I would assume that even if Ampire might work in FL Studio, it probably won't run as well on 8.1 as it would 10.