A:Answer I was hoping Lenovo would answer my question, since they make the motherboard, but no such luck. So I'll provide a partial answer myself.
In the end I decided to replace the two 8 GB memory sticks with two 16 GB sticks rather than adding the new memory to the old memory . The reason is that, as much as I hated giving up the extra 16 GB, filling all four slots would make the memory run slower.
For computers with four memory slots and an Intel 12th Gen Core Processor, the maximum supported DDR5 memory speed is 4400 when two slots are filled. If four slots are filled, the maximum supported speed is 4000 for single-rank memory and 3600 for dual-rank memory. That's an Intel limitation. I've read on the Lenovo discussion boards that filling all four slots results in worse performance that that.
(The single versus dual rank in not generally even mentioned in memory stick specifications. I read that if the chips are all on one side of the card it's singe-rank, but I'm not certain that's true.)
In case anyone wonders, I ended up using ADATA - XPG Premier 16GB 4800MHz DDR5 U-DIMM Desktop Memory, and it seems to work well, running at the maximum 4400 speed.