Once a fare bucket is purchased on Frontier (Classic Plus, Classic, Economny) there's no way to buy-up to a higher fare level, neither with dollars nor points. So nothing you did could have forced a charge (dollars or points) for an upgrade to Classic Plus. (As a matter of fact, I think the ability to buy-up to a higher level is something that should exist as an option, but it does not.)

The only sort of upgrade possible after purchase is to move up to a Stretch seat. However (1) that is pretty clearly defined as an additional charge you must deliberately pay for (2) chosing exit row is not Stretch, and (3) even if you had upgraded to Strech (which you did not because exit row is not Stretch), it does not change your ticket to classic plus.

So what happened? I'd guess that the fare class purchased (I don't mean classic plus/classic/economy, but industry-standard fare bucket like Y, B, M, K, Q, or V) automatically qualified you for classic plus even though you were booked at the lowest fare level available. In some instances...either when the advance purchase time is short or when something quirky is going on...the classic plus fare is equal to (or I've even seen less than) the economy fare.

Bottom line is, however, becuase there's no mechanism by which you could do a paid upgrade from economy to classic plus, either with miles or dollars, I don't think there's any risk that your selecting an exit row seat lead to any additional charges.