Technically you should reset the BMM, but in reality you can just swap them over and I bet most are without giving it a second thought.

The BMM monitors the characteristics of the battery to optimally charge it, and determine how quickly it will discharge so the car switches off non-essentials earlier as the battery ages. If you just swap the battery over without telling the BMM it will just treat the new battery the same until it has re-learnt the characteristics which takes a set time/distance.

For example if your current battery discharges quickly you may notice the "Battery low, start engine" and "System will shut down in 1 minute" message soon after ignition without the engine running. Change for a new fully charged battery and that message may still appear for a few trips until the car works out it can stay on for a longer period.

You could avoid that by leaving the car connected to a battery conditioner for a few days prior to swapping the batteries over. Or just not bother - nothing will break, at most it would be an inconvenience for a few days if you like to listen to the radio with the engine off.