If you connect a post to a sill with a tenon, the mortise is created in the sill as well. If you don't explode the joint then the mortise stays linked to the post and if you move the post the mortise will move with it.
If you convert the mortise into a number four function then it can be transfered to the post, and where every you move the post the mortise will go with it, and when reprocessed it will re-cut the mortise. That way you could place the post in the library and bring it back later.
To attach the mortise to the post you'll have to explode the joint and delete the mortise and then recreate it using a notch function. Then the notch/mortise can be transferred to the post as the notch is a #4 function.
It really depend on exactly what you want to do. If you want to be able to move the post in this current frame just don't explode the joint and it will stay linked to the current sill it is in. If you move that post out of that sill then the link is broken and you'll have to redo the tenon to a new sill.
If you want to put the post into the library for other frames then you'll have to link the mortise to the post by the method I mentioned above.