What’s not clear to me is whether there is anything I need to do from the OS/firmware side once I replace the memory, or will the new parts get automatically recognized? (I use Qubes OS.)
as long as you put in the right ram no adjustments should be needed. i for one have ddr4 with 2100mhz and one with 2333mhz inside, i guess the frequency stepping is compatible, so in my case i could use ram i had from another machine - as you can see very compatible and high chance everything runs normally. qubes will take what ever it gets, no config changes necessary