Ressource fonction Date

There is a strange issue in the way we use fonction. We can't fill the date field start and end.

The only way is to create the same fonction on the ressource, but how do you do if an "developper" become the "lead developper" ?
The two fonction can exist in the same time but project line will appear twice.

My reply proposition is in the picture.

We can't delete all of his assignment roles because he got input on his role for the related activity
This suggests another problem, we want to keep historic for billing.

I don't understand why those date are unavailable and the only way to fill it, is to increase the cost of function.
I think that the function could be end just for one ressource and one cost, without touching historic of the ressource.

Example :

My ressource called Henri has 2 functions :
- Manager (For the "A" activity)
- Developper (For the "C" activity)

Another ressource called David has 1 function :
- Developper (For the "C" activity)

Henri has to leave the "C" activity because he become the Marketing Director of the A activity and he does'nt have enough time to fulfill both functions.

How do you do this on projeqtor ?

If we could just write an end date for Henri, he won't see anymore the "C" activity in hist input and we should keep all of his historic at the right cost. (this is very important for our report)

In the other hand Henri got an upgrade of his function for the "A" activity, we dont want to see duplicate rows on his input for this activity. Fill the end date could just fix this.

Sorry for my bad english, it's not simple to explain what i thhink about.

Thanks for reading me.