Is it possible to gain back the Fire type after using Burn Up?

Question

Let's say my Moltres uses Burn Up. It then loses the Fire type. Then I switch to a different Pokemon. Then, I switch back to Moltres.

Will Moltres gain back it's Fire type upon switching in, or will it be a pure flying type for the rest of the battle?

Answer 1

Burn Up, just like Soak, Forest's Curse, and any type-changing that results from abilities like Protean, is a temporary flag that is lost if the Pokemon ever moves out of an active position. The next time it switches in, it will be back to its original type.