I somewhat like the 'reinterpreted' rule, whether that was what was in the mind of the creators of the game or not. That would be:

There are no restrictions on changing from battle sails to full sails, except that you can't be missing a rigging section.

If you didn't move from your bow hex in the previous movement execution phase, then your movement is restricted by
1) you have to move one hex straight before you can turn (already in place)
2) you lose a turning factor (this too)
3) if you have full sails, you can only use your full sails movement allowance if you go straight ahead, no turns. Otherwise you'll have to use your battle sails movement allowance. (new)

This means that ships already on full sails would be restricted by (3), whereas currently they are only restricted by (1) and (2).

I have no idea if this is tricky to implement or not. If too much trouble, then I am with Electro and vote for just ditching (3) altogether.