What happens if you reach a new progression mid event?

That's my straightforward question. For example I will finally become elder on August 20 when the last boss is available. The upcoming Revenge event will start on August 12, what will happen exactly?

If I take part in the event and I play the quests and stuff will my progress save? Or will it reset?

Should I play in the event or should I wait till im elder just to be on the safe side?

Im especially concerned about the free keys, elder will get 9 free keys and valiants only 7.

How will that work when I become an elder? What do you recommend I do?