I’m pretty sure you’re going to have to use a live USB. You can’t modify a partition while it’s mounted and you can’t unmount a partition that’s in use.

You would just install one of the KDE meta-packages. After that you can select the new desktop environment when you login again.

The only thing to note is that some settings don’t always play nice between the two (for example one might overwrite settings on the other), so maybe consider making a new user account just to see if you like it.

What desktop environment are you using? KDE let’s users do this pretty easily by just going into the color scheme settings. I’m not sure what the process is for other desktops these days.

Is it 32-bit? If it is, then that also severely limits your options. Personally, I just throw Debian or one of its derivatives on old hardware like that. You may want to consider Q4OS. It’s Debian based and is geared specifically towards old and low-end hardware. Interestingly, it’s also one of only a couple distros that ship with Trinity as a desktop environment.

Retro gaming is definitely doable with 2GB of RAM, considering that older Raspberry Pi boards can do it with just 1GB. In that case you could try Batocera.

Some other ideas include running something like Nextcloud or a media server on it on your home network. In that case, I’d again recommend Debian.