In a past post someone said just use a Do-More! OK how about this.
I had to keep most of the systems running so it was a multistep process.
Move the 450 rack two out of the way.
Install a temporary 450 PLC where rack two was.
Consolidate the two 450 racks into the temporary 450 PLC.
Install the DM, write the program and cut the loops over one at a time.