Dell Optiplex 9020 and TPM and CPU

Re: is it possible to upgrade the CPU from a 4th Gen to a newer, more up-to-date CPU, or, would I also have to update my motherboard too

yes you would need to update motherboard to support a cpu to run Windows 11.  cpu needs to be at least Intel 8th gen or equivalent level.  there is no Dell optiplex motherboard that can fit 9020 case because modern optiplex (and other Dell desktop model) motherboard has become highly proprietary.  your option includes non-Dell microATX board with 300 series chipset or higher, such as H370,B350,Z370.  You will need to upgrade psu to standard 24 pin too because 9020 stock psu uses proprietary 8 pin.

TPM will be addressed after you choose a newer motherboard.  But ask yourself whether you really want to keep using the 9020 case, or just get a new complete pc.

Thank you so much for your reply, it certainly has given me food for thought, it does look like a new computer sometime in the future, I don't think it's worth buying until we know a lot more about what is needed. Things like this do have a profound effect on pensioners like me, if I start saving now, I could perhaps afford a new pc by the time Windows 11 comes out properly.
Thanks again.