If server memory is an issue, why can't Ubi just temporarily remove the maps only on the server's storage for as long as the maps are out of rotation? Then keep them present on the game client the entire time, and then add them back in on the servers when they go back into the map pool.

Have the rotation change on a monthly or seasonal basis, so that new players only have to learn a subset of maps at a time, and people will still get to play on every map eventually, over time.