GSMs engineering for Longhaul is Monarch and our A/C are very reliable, as I am sure LS A/C are well maintained. Don't really think it matters what age an A/C is as long as it is adequately maintained.
If the B752 is leaving the fleet, it's because there is no need for it this summer. There is an extra B763ER coming in Feb - all economy. This will allow GSM to sell more seats to YHM.