Instant upgrade fares do not require stickers. But, like all discounted / low fares, they may be taken up by others as time proceeds and no longer be offered on specific flights, and they may have advance purchase requirements (so attempting to purchase one after the 21, 14 or 7 day advance period has lapsed means they are no longer available at the lower rate).

Other issues affecting fares are season, both the IATA season (IATA members have season conferences at which schedule and fare changes are discussed), and demand - Revenue Management is a dynamic, computerized system used by today's airlines based on predictive analytics algorithms and actual sales to modify available fares, awards and seat allowances.

The OP may be dealing with season issues - Fall is a high demand / more expensive time for BOS, winter probably one if one is fling to MIA. And fares have also been sneaking up - based on demand and available seats, and most airlines have reduced capacity in recent months. Sometimes selecting more results when booking can reveal lower fares that are connecting or direct routing, rather than nonstop.

It's a complex issue, with fares changing for a variety of reasons - it gives FT a reason to exist, so it's not all bad, right?

Footnote: heading to BOS later this month, and yep, ex-West Coast, BOS fares for instant upgrades are now higher - maybe they should cut down all those maple and other colorful trees and we wouldn't be bothered with high fall fares!