I don't think that it worth considering upgrading 18 bit (22 bit?) speakers. When I last looked the cost was comparable to buying original 24 bit second hand.

I had a similar problem to you and upgraded a 565/596 to a G68/G98 set up when they first launched to get the HDMI link to my projector. I stuck with my 18 bit speakers and still thought that the sound was a worthwhile upgrade. Last year I saw the chance to update the front to DSP5200. If I ran the upsampling in the G68 I lost sound from the rear speakers. If I left the upsampling out the front speakers upsampled. It was another good improvement. Fortunately I then got the chance to replace the rear 18 bit with DSP3100. I know others say you should have the same speaker all round but I think it is a good compromise. I always thought that full size rear speakers were rather wasted.

This year I have added an HD621.

My advice is to do it in stages. I am sure that you will appreciate each upgrade.

The 18 bit speakers still give great service. One pair is connected to the 565 and for the other I found a second hand G91. It's Meridian all round!