You would need to replace U2 (SRAM) and U3 (flash) on the device, according to the schematic: http://ftp1.digi.com/support/documentation/0900177_k.pdf
And then you'd need to run the WriteIDBlock program to identify the module as an RCM3720 instead of an RCM3710. You'd want to make note of the MAC address of the device so you could re-use it in the new ID block.
Depending on the cost of those chips, and the cost of the skilled labor to replace them, I guess it could work to go down that path. It sounds labor intensive and obviously voids any warranty remaining on that hardware.
You could also do a bulk order of RCM3720 modules, install your code on them, and then ship out to users as a hardware upgrade of the product.
You could potentially do that for some of your users and ask that they return the pulled module, then perform your hardware upgrade on the returned hardware and ship it back out as a replacement for other users.