That happens routinely on F9. When there is an aircraft change, or the exit row # changes, passengers are randomly reassigned seats by the computer. It starts with row 1 and moves back, so you were assigned a "stretch" seat at no additional cost. Some stations will manually reassign you if you aren't entitled to it, but most won't.

If one leg of your trip was Classic Plus, even the second leg of a connection, you will be classified as Classic Plus for the whole ticket.