Flying Blue (Air France & KLM) - Fare or Price?

I noticed that the same class fare (Z,D,... or V,K,H, ...) change price from on period to the other. for exemple L fares are more expensive during the french vacation than sometime else.
Am I right? in this case can I book a flight in L for a given inexpensive date and change it (as a youth ticket booked in france that is change fee free) for a more expensive date? In the end my question is do they take into account the price or the fare class?
Thanks

IME they do.

Where it gets a bit complicated is that tickets are not sold by fare bucket (P, F, J, C, etc) but by fare code (e.g.ZHAPGB, ZAP14GB, etc) which all book into a given bucket (e.g. in the example above, both fares book into the 'Z' bucket but are priced differently). Each fare (the long code) comes with its own conditions. For instance, I bought tickets from LHR to JFK in Z for which changes are priced at £200 each, while some other LHR-JFK fares in Z have no fee for changes. Other conditions (such as travel dates, minimum and maimum stay, or even which day of the week they are valid) can change by fare code so you need to check the specifics of your fare code and its restrictions. Often, you will be allowed to change the date but the fare may also be recalculated if the ticket needs to be reedited even if the change per se is free.