when you switch from one number to another, do you have to open and close the top before, after, or not at all when changing numbers? or...do you just leave the top closed and just change the setting any way you want?
Always open (or at least loosen) the doors before making any adjustments. If you try with the doors closed, it puts a lot of tension on the adjustment mechanism and you risk bending or breaking something.
The mechanism was designed in such a way that you don't need to undo the doors.
In a fully functioning razor the only force that can possibly be applied to the adjustment thread or plate is that of the spring.
However if the spring is jammed, plate jammed or the adjustment threads are already very worn (Unlikely when looking at the materials used unless someone sat winding it back and forth for years !) then it may be best to loosen the doors.
Of course, if jammed, it should be just thoroughly cleaned and freed off anyway.
Just my 2p's worth YMMV.