This is a great question and one that everyone will face when we reemerge to the water. In the end, we want to start conservatively. Starting where you would be at the end of the off season is a great starting place. Swimming is the first sport where you will loose conditioning because it is so technique based. If you start conservatively and reacquaint yourself to drills (and the water in general), your conditioning should come back fairly quickly if you kept your swim strength in place during this time. With a gradual build up, you should be able to get back to a place to transition back to your plan.
In the end, swimming is all about technique. So when you reintroduce yourself to the water and gradual build back up, a strong focus on technique is key. Thank you for introducing this question for all to learn!