I would hold fast for right now. Leonard is not droppable in any format in my opinion. When he gets back he should step right back into a major role for the Spurs, and his all around production is valuable. Seraphin seems to be coming into his own, even if it is slowly. He will be inconsistent, but I like his upside better than the FA's. If anyone I'd swap him for Jason Thompson, but I think a month from now Seraphin will be outplaying JT.
Mullens could end up being droppable IF his demotion to the bench sticks, and Biyombo/Haywood take the majority of his minutes. But a 1 game sample is not nearly enough to drop someone who has been valuable in multiple categories so far.
I like Jordan Crawford and Shannon Brown as far as guards go for your FA's, Danny green is the best SF and Morris/Thompson are both decent big men, but I wouldn't make a move quite yet.