Have not flown Frontier before and have a question about what one does with a personal item when seated in row 1. I had purchased a seat in row 2 but the flight along with my seat assignment has been changed. Thanks!

While Frontier charges for "carry-on" bags and requires that your "personal item" fit under the seat, as far as I know there is no requirement that you place you place your "personal item" under the seat. In fact since Frontier charges more for a "carry-on" than for a checked bag and since many people are perfectly fine with traveling with a back pack as their "personal item" the overhead bins are usually almost empty. I've never seen an FA tell anyone that they could not put their "personal Item" in the overhead bin.

Re: OP. If in row 1 they will make you put any personal items up in the bin for takeoff and landing. Very strict about this. On most planes the frontmost bin is all or partially equipment, and the FAs save the second bin for row 1 even though they have to vigilantly guard that and tell people in other rows they can't use that. I usually get row 2 and know how the whole thing works, but most people still don't seem to.

MOST of the time if you're in other rows you can put your personal item up in the bin...FAs really don't like to be strict enforcers of these newer baggage policies that are there mainly for revenue (FAs are mainly there for safety, remember).

HOWEVER, I've noticed on some long hauls the bins can get close to filling with paid carry-ons. When that happens the FAs can sometimes pull smaller items down and ask whose it is because they don't want paying customers complaining. Most of the time this doesn't happen and it's nothing like the madhouse on the legacies (where if you're in a later zone the bins will be totally full and anything not fitting under the seat gets force-checked). But some flights do get a lot of paid carry-ons and lots of people ignore the sizing limits if they're paying.