I deleveled from Rank 8 back to Rank 7 today by switching to Windurst and losing Medal of Altana.
This provided an opportunity to test exactly when your rank updates. We know that the packet containing Rank gets sent every time you open the equipment menu, but it doesn't seem to update then.
I got Medal of Altana back and
* Checked my rank, still 7
* Opened the equip menu waited for the loading to finish, checked rank, still 7
* Went to the campaign warp NPC, checked rank again just for fun, still 7
* Zoned to Xarcabard [S], checked rank, back to 8
So TIL 2 things:
* Mastery Rank updates when you zone
* You can delevel
Also, I haven't stopped grinding titles / accomplishments since I got Mastery Rank 8. I also got ~5 jobs to MLevel 30 and capped their skills. My playtime is obviously higher now than it was then. I've gotten 8 new Titles since I ranked up to 8.
* Winning Owner
* Victorious Owner
* Lumber Lather
* Shell Scrimshander
* Bone Beautifier
* Knitting Know-it-all
* Glorious Groomer
* Dread Purger
All of which seem to have been either worth less than Medal of Altana or some level is gated by having Medal of Altana.
Edit: Just going to keep adding stuff here until I rank up:
* Curator Culler
* Tumult Curator vorseal
* 1 synergy level (this is so boring)
* RUN to M.Level 30
* Prying Eyes 1 (W) - 98% last windurst one done