In general across airlines nowadays (I believe DL was among the first to move to this model about a decade ago) flexibility and such is not encoded into the fare class, but there are fares with different levels of flexibility/advance-purchase/etc. within most/all of the classes, with the less restrictive fares being priced at a premium to the more restrictive fares in a given fare class. The fares also increase in price for a given level of restriction from class to class... a fairly common practice is to have the dollar amount of the premium for less restriction increase as one moves up the classes but for the premium as a percentage of the cheapest ticket in the class to decrease (e.g. a V light fare might be 100 euro while the V flex fare is 200 euro, but the R light fare is 120 euro with the R flex fare at 230 euro).

This is generally a better situation for all: the airline gets finer grained revenue management and passengers don't end up paying that much extra for flexibility (instead of having to go up several fare classes). Buying the most flexible fare in a class doesn't give you any extra mileage in programs that use fare class as a proxy for how much you paid, but you'd typically pay at least 3x for the fare in a class that gives 2x the earning rate.

Note that if crediting to FB, the mileage earn is a function of the fare paid anyway.