Hi Schultz!!! dont worry about ships reverting back to 1938 ect when you click on properties tick the box evolve from entry date.
When I write Missions in 1940 and I put a blackswan in the Convoy Escort Screen blackswan as no Hedgehog mortar bomblets behind front gun turret when I write a June 1943 Mission Hedgehogs are on the blackswan Escorts but you must tick Evolve from entry date box.
Also if you are putting convoys into the mission make one of the ships a group then add other ships to that group then click on evolve from entry date on the group leader then all ships in that group have evolve from entry date ticked by default then just add way points.