New PC: upstairs in the office
Old PC: downstairs in the guest room

How to switch? Open door, walk upstairs 😉

And somewhat less facetious: I have multiple "old" PCs downstairs including a pretty recent (8th gen Intel Core platform) Linux machine for downloads and looking stuff up online etc. They're all hooked up to a KVM, except for video on the recent machine which has its own little 16" TFT (SGI 1600SW 😉 ) next to the CRT for the rest. Audio is mixed together with a Behringer Eurorack mixer, MIDI goes via a patchbay.

I have a wide desk and they are side by side. I just slide my chair over, or sometimes just position it between them and lean my body towards the one I'm using. 😀 There is another retro PC on another desk in the same room that I seldom use. But it's only 10 feet away if I do want to use it.

I just use a KVM switch. 😜

Having a PS/2 to USB converter also helps: https://www.tripplite.com/ps-2-to-usb-converter~0dt60002