What does it look like and how would it impact the value of a paper note?
Back in the day, collectors desired perfectly flat notes, so embossing and natural paper waves were “ironed” out. Fast forward and collectors changed their ideals to obtaining notes with original paper qualities, including embossing and natural paper waves. Re-embossing the seals and/or serial numbers is tedious work involving a stylus and can be very hard to detect. I’m not certain exactly how third party graders can spot this handiwork and other minor improvements like trimming, but they do. If two similar notes came up for auction and one was original and one was re-embossed, I would suspect the original note would sell for more. Here is an example of original embossing on a modern $2 note. You can feel it with your finger tips and see it with side lighting.
Here’s an auction listing with a notation mentioning re-embossing. It’s really, really hard to detect once the note is encapsulated. https://currency.ha.com/itm/small-size/federal-reserve-notes/fr-2010-d-10-1950-federal-reserve-note-pcgs-banknote-choice-au-58/a/142130-84478.s