Step 1 schedule is built around First Aid's layout which means that each year has a slightly different schedule. Because of this, it's not possible to switch your edition of FA once you have built a schedule.
What we generally recommend is that when you get to the end of a subject block, you can just build a new schedule and start with the new version of FA from there. Since you can have multiple schedules at once, there is no need to delete what you've done previously - just create a new one where you left off.
You can edit the schedule so the dates are exactly the same as the old schedule and it should basically be the same thing.
The checkboxes won’t be there for the weeks you've completed in the past, but fortunately, it won't affect the future of the schedule in any way.