what is the best solution to replace PM633 CPU with AC800M in the following Configuration
you can reuse existing S800 IOs. All CI810 must be replaced by Profibus interfaces CI801 or CI840, depends whether redundant AF100 is used for IOs now.
How many pages are in the PC program and what is the program scan?
I would say PM861 should be enough for PM633 replacement. It would be better to test it before you issue detail design.
Investigate which communication protocol is used by CI532 in AC160.
CN3 port of AC800M or CI853 can be used for Modbus RTU. Try to upgrade serial communication to Modbus TCP if possible.
Try to use Ampl2m online webtool for logic bulk translation.