Do Marines have 30 days after promotion to have new chevrons sewn onto their uniforms?

Sgt (Join to see) I have also looked at MCO P1020.34g, and could not find any mention of a grace period. Have you asked the 1st Sergeant?

You are expected to wear your rank from the time a CO pins it on and your promo warrant is read. As to sewing stripes on, when I was in you either displayed the proper uniform for a wall locker/ JOB (junk on the bunk) inspection or a laundry receipt showing they were turned in for stripes to be sewn on. It was best to ensure you could wear the inspection uniform with the proper rank. Since you know a few days after making the cutting score or being selected on a Meritorious board, the first thing you did was turn in one uniform on a rush order and the rest to follow. The 1st Sgt usually had stripes in his wall locker and replenished them from Bn Supply. Good luck finding that in any MCO. It's just one of those things "Such as Regiments Hand Down Forever". There are many things that are just passed on, such as the junior enlisted eating first and no seconds until everyone has eaten. I'm sure you can think of many more. Congrats on your promotion.

where do you get the new ranks applied? How does the promotion process work?

WHAT? Your kidding me that is a thing? Even when I have seen special promotions, someone has made sure that individual had a duty uniform to wear. I have never heard of anyone not having their new rank on the day of promotion. I can't see it happening in any branch. My shirt would have been, "if your not ready to wear it, we can make sure you don't have to."

Being that there is no written official rule, it is whatever timeframe is within reason according to the individual's superior, who has enough authority to do what is needed in the case of non-conformance for improperly wearing the uniform . Any military member should be treated as the rank they wear. This is also why there are rules and laws governing the propper wearing of a uniform and its against UCMJ to impersonate, etc, etc. I would address said individuals for the rank they are wearing. If they don't like it, they'll get it fixed.

Why would any Marine procrastinate that long!?!