In my experience, as long as you change all the batteries as part of the pack at the same time you will be fine. The pack should be the chassis batteries as one pack and the coach batteries as another.
If all of your batteries are tied together, then I would consider that one pack.
I changed just my chassis batteries a while back with no issues.
The batteries being charged in a pack should be of the same age, they tend to last longer because they charge similar as long as they have been maintained.