I would buy the new MSFS 2024 but only keep the old one. Or what exactly was your question again?
¯_(ツ)_/¯
But seriously: technically nothing speaks against having both installed on the same machine (for what we know so far). E.g. you will have two distinct community folders, so no conflicts expected there.
As I type the Samsung 990 Pro 4TB Nvme drive is available here and there for under $300. A worthy addition to most systems.
I will have both installed, but will also minimize the footprint of 2020 by deleting most of the WU and CU. Or I’ll just keep it on the Xbox. 57 days to decide!
On Xbox, I purchased 2020 while on pc so will keep it loaded. I have no intention of purchasing 2024 will on fly on it when I have game pass