A while ago I upgraded my Commodore 128 by installing JiffyDOS. This is a fairly straight forward procedure - obtain EPROMs, burn the (legally obtained) JiffyDOS images and swap out the C64 and C128 kernal ROMs with the new versions. I wanted to be able to switch out JiffyDOS, just in case, so I used EPROMs twice as large and installed a switch that would pull the right address line high on each IC.
Yeah, I'm not really sure why I used two pull-up resistors - the switch controls an address line on both ICs at the same time, so that's kind of wasteful. Anyway, it worked fine.
This Commodore 128 is an older model, the kernal and BASIC ROMs are the first revisions and not the later, bug-fixed versions. But you don't install the old versions when you replace the ICs, do you? That would make no sense. By upgrading the kernals I now had a Franken128 with new kernals and old BASICs. That's easy to fix, just replace the two BASIC ROMs with two new EPROMs, right? Right.
A better way
But it turns out there's a better way. The Commodore 128DCR (128D, cost reduced) only uses two ROMs, not four. The old 128 also supports this mode via jumpers - almost. There's a slight hardware bug, but that's easy to work around.
Two EPROMs instead of four? Sounds like something I want to explore.