Recruitment is the beginning. The journey is the journey.
Around mid- to late winter, after having the time to review all of the reports and conduct interviews, Gwendolyn gathers all of the mice of Lockhaven together and presents the year's promotions. Occasionally, the ceremony is broken down into two days—one day for tenderpaws and guardmice, and one day for patrol guard and patrol leaders.
-- Mouse Guard 2e, "The Winter Session", p.160
So! It's winter in Lockhaven, and Daisy the tenderpaw has had a busy year. She's run around with her patrol, logging passes and fails on all kinds of tests, maybe starting to pick up a new skill here or there. She's found a new friend! She's made a new enemy. Maybe she's had to tax Nature enough to permanently drop it a point in order to succeed at Guard business or stay alive, maybe she hasn't.
Regardless, Daisy's patrol leader feels like she's put in enough to be promoted to guardsmouse, and the GM agrees, so it happens. There's a nice ceremony, the patrol leader picks out a new cloak for her, it's adorable.
Now to the business of mechanics. Daisy's player takes an eraser to their character sheet, rubs out "tenderpaw" by Guard Rank and writes in "guardsmouse". They write in the cloak color Daisy was just awarded.
...and as far as mechanics go, that's pretty much it. Daisy was promoted as the mouse she had become during play. Her player doesn't have to go back to the Recruitment section and make "Daisy the guardsmouse" with a new set of basic traits and a fresh skill distribution. "Daisy the promoted tenderpaw" differs from that theoretical "Daisy the guardsmouse" exactly because of everything she's experienced.
One special mechanical thing does happen because of the promotion: Daisy's advancement to the next rank of Circles and Resources, all the passes and fails, are cleared, and she just jumps to the next rank, as word of mouth and a fatter pay packet do what practice alone could not.
But there's a pause in Winter to reflect.
Two more important things happen during a conventional Winter session that I'd argue you should always do even if the mice find themselves going on a winter mission - they give up the chance to recover and train and pursue unfinished business, but those two things are intended to wrap the entire year's progress and it's not like you give up your past to go out during Winter. They stand apart, a bit, from the scratchpad of consequences that is marking down checks by skills.
The patrolmouse reflects on the year as part of writing a yearly report to Gwendolyn, and as a result, they get to add one Wise that stands for the thing that year taught them the most. And, the patrolmouse's patrol mates reflect on the year for the patrolmouse and as a result, the patrolmouse gets a new trait, changes or elevates an existing trait, and changes or removes a third one, based on what their patrol thinks the year brought out in them.
And Nature isn't a tradeoff for anything.
During Recruitment, the Mouse Nature questions give you the option of trading in three points of Mouse Nature for two level 1 traits and one rank in Fighter, but all mice have equal access to those regardless of rank, and only during Recruitment - where a mouse falls in the range of 3 to 6 Nature has more to do with the inclinations of the mouse than their rank in the Guard. Since you're not redoing Recruitment when you advance, that tradeoff in Nature happens once.
It is true that having a lower Nature makes it easier to learn new skills, but acquiring new skills and ranking up existing ones doesn't do anything to affect Nature either. Nature advances like any other skill, through passes and fails, and the only exception carved out for it is that since Nature tax is recoverable, you always advance Nature and judge if you've had enough Beginner's Luck by the highest recoverable value of Nature.