I have tried at least 10 different combinations of trying to downgrade to 3.15 on that slim machine. If I can dig up the post, it'll say most of what I've tried.
Here is the post:
http://psx-scene.com/forums/623011-post592.html (TI-84 Payload Collection)
I ended up trying all the combinations in that post and nothing ended up working for me. It's not a problem, since I can still get back to 3.41 retail, but I just wanted to try some new things out.