pro/program and interchange assembly

I have the following question.
I would like the model to promt the user for a choice, ie: "Do you want
A " (yes/no) and then to react to the input. This point I have created a
interchange assembly for the two components A and B and can manually use
the replace command, not sure is this is the right way to do it.

Now the I think that I got the input and promt parts down but how do I
make the model to change the components as I regenerete the model?

If question = YES
add feature A
end add
else
add feature B
end add
end if
?
And what is the proper syntax for the id:s and such?

I would really appreciate if someone would be so kind and point me to
right direction.

Cheers, Antti