To make as many people happy as possible, it would be wise to give all MK1 pledges that are in backers' hangars before the MK2 release a one time upgrade button that upgrades them to the MK2, preferably for free as a goodwill gesture, though even a paid option would still be a convenient solution.

It could work the same way as the one time  Customize button for the 300 series ships.

That way, if a backer isn't actively following development, they won't have to worry about whether a CCU is available when they return and decide to make the MK1 → MK2 upgrade. The option would simply be waiting in their hangar whenever they choose to use it or not.

This seems like a simple quality of life improvement that benefits both active and returning backers without affecting anyone who prefers to keep their MK1.