FAb7D If I understand correctly KDE and Budgie cannot coexist
The best way is simply to do a clean install of the one you want to use, and stick with it. When I first learned about Solus, I naturally used Budgie, because it was touted as the "flagship" DE of Solus. And it was very good. That's when I stopped "distro hopping" and I've been a Solus user ever since. But curiosity eventually got me to try all four Solus editions, including KDE Plasma. Once I found out what I'd been missing, I've never looked back.
If you're unsure which would be the better DE for you, I'd recommend creating a virtual machine of the one you're not currently using as a daily driver, and work with it in your spare time. Both DEs are very happy running on a virtual machine, and VirtualBox is very easy to use. So long as you're not using Wayland as your window manager, you can use a VM at full-screen and experience the DE almost as if it were on the hardware itself.
There is a slight performance difference, but for most uses it's something you can easily ignore. And for activities that require the most resources from a computer, you can easily shut down the VM and use the "bare metal" host system to do the work. When that task is finished, it's easy to change back to the VM.
I'd explain the reasons why KDE Plasma is my daily driver DE, but that's not why I answered you. I'm not here to get you to change from a DE that's working well for you. I just wanted to explain why the best way to switch from one to the other is simply to run both at the same time -- one on your hardware, and the other in a VM.
Incidentally, the problem with running a VM at full-screen in a Wayland session may be a temporary thing. It's a bug that's been reported, and hopefully it will be fixed before long.