Well, you can always upfare. Meaning that you "change" your ticket to the same date/flights in a higher class of service. You'll then also get the additional miles. But the cost would, of course be the same as if you had straight up purchased the business class fare.

Just for reference - I flew AF to SJO a few months ago and the upgrade offer (From premium Economy to Business) was over 1000 euro one-way. This may be different for you, but it does not have to. Last minute upgrades are not always cheaper than just purchasing a higher cabin fare.