Huh, I was expecting this to be about T'lyn! no such luck on that front unfortunately. Haven't combed through the trailer in too much detail, but if this is the first time I'm hearing confirmation about them all getting promoted would that imply most of the trailer shots are from relatively early in the season?
It feels right for the gang to get promoted. If you asked me what Lower Decks is about, underneath all the nostalgia and jokes, I'd say it's about growth--Starfleet is awesome, but its legends of yesteryear don't start that way. These goofy, lower-decker ensigns may be screwups, but at the core they're Starfleet though and through. It kind of feels appropriate to move on from where the original TNG episode Lower Decks left off-- the promotion of proto-Boimler Sam Lavelle-- and explore what growth looks like beyond that.
Edited to add: I went back through the trailer and yep, no visible changes to their rank pips. Although T'lyn has a shot where she's switched over from a provisional lozenge to the standard single round ensign pip.