I've worked on a few strapping machines in my time, including monitoring the PLC program for the strapping head. I've never seen an instance where online programming changes were necessary or advisable for the strapping head.
Because the same RSLogix 500 tool is used to program both SLC and MicroLogix 1500, and because the base + CPU only adds two part numbers to your inventory, I'd recommend going with the OEM's plan of a controller just for the new head. They can program, test, wire, and ship it as a unit.
There may be a high-speed counter or other reason for them to want to use the MicroLogix 1500. The onboard I/O is an order of magnitude faster than a RIO link.