I think changing the power supply is a good idea as the frequent battery failure is unusual.
You will loose the PLC memory on changing the power supply so make sure you upload the program. Incase you have a flash memory card then the program can be reloaded from the flash memory but the memory bit and actual values of the DB will be lost and set to initial values. If you are sure that the program on the flash memory is a working program then you dont have anything to worry about.
After changing power supply, the CPU will go into a fault on first startup requesting memory reset and then take a couple of minutes to go to run as it will do a complete memory test.