WFP does really push RB3D to its limits. For example the usage of the slots available for the planes.
In fact, RB3D doesnt has enough slots availabe for all planes WFP uses. As a sollution the slots are occupied by multiple aircrafts - of course at different dates.
If a plane is withdrawn from the front a new one can occupy its slot without any problems - even if its an enemy one.
The only problem which could occur is by changing your pilot/campaign. For some missions the game date remains the same as in your old campaign. The result may be a german squadron full of Nieuports

Thats quite confusing for the first time...