I foolishly went for Trauma difficulty first, continuing my misguided policy of always starting a new game on the hardest available setting. I didn't know until too late that by doing this I miss out on all of chapter 5, Demon Morph, certain cards and levels.
Now that I beat Alastor, I don't see any option in the interface to replay levels and different difficulty levels. Trying to "Sign the Pact" warns me all levels will be relocked, and I assume I'll lose all my cards and weapons also.
So, now what?

I think you will get a message after you sign the pack if you want to reset your cards or not. You will however lose all your weapons if you play an another difficulty.
You may want to wait for someone else to confirm this though as I played this game in 2004, I did play overdose this year but thats a published mod.

That's correct. You lose all your weapons, and all the levels get re-locked, but you can keep all the cards you've unlocked.

Ah, k. Now I feel silly that it was so simple.
I'll collect a few more cards in Trauma then before backtracking to Nightmare. I'm told some of the last levels are draw-droppingly gorgeous, so I wouldn't want to miss them :)