I have the Action, Weapon and Condition cards from the Mouse Guard box set (first edition, in case it matters). I want to run Torchbearer, which is based on MG. Can I just use the Mouse Guard cards to help my players keep important information around, or have the rules changed too much?
Action cards are fine. The interaction matrix between the four different actions, and their basic obstacles when they are checked through Independent tests, has stayed the same, so you can continue using the action cards, and they are probably the most useful of the decks in this context anyway. (Note that the Mouse Guard Box contains 3 full decks with 3 cards for each action, but in Torchbearer, you will have at most two teams in conflict with each other.)
Some Conditions have changed. This is not just in name (tired → exhausted), but also in number and effect. The Fresh condition was added, and is useful to keep in mind for a party because it gives +1D to all tests, but it does not have a Mouse Guard equivalent; Neither does Afraid. Hungry and Thirsty behaves the same in Mouse Guard and Torchbearer, and Injured and Sick each gained an additional (minor) effect. Tired became Exhausted, and both it and Angry have seen significant change in their mechanical effects.
Weapons have changed. Torchbearer contains a lot more different weapons, and while some remain similar (axe → battle axe), the types, effects and use structure of weapons in Torchbearer are widely different from Mouse Guard. Don't use those cards.