There is no official guidance on this as of now. However, if you wish to speculate.....

The Q400s are not going away overnight, and the replacement E175s are not arriving overnight. The last of the Q400s will be gone at the end of summer 2016 and the E175 deliveries for (for UAL flying) will go until August 2017.

Whether displacements from the Q400 occur is entirely dependent on the rate at which pilots leave the equipment voluntarily. For example, if every Q400 CA bids off for the jet via a vacancy, there would be zero displacements. In fact, if too many Q400 pilots bid off via vacancy, they could theoretically have run some Q400 CA vacancies. This is occurring now on the E145, even though all the contracts will expire by the end of 2016.

And the company benefits TREMENDOUSLY from vacancies because they would not have to pay those pilots moving expenses.

Inevitably, some Q400 CAs will wait to be displaced since they might hold better relative seniority in base on the Q. I am already seeing a TON of Q400 CAs bidding off via vacancy and going to class for the E170 at YX (not S5). So you will see guys going to both certificates.

As for the Q400 FOs (as I suspect you are most interested) the company has them by the balls. The only way they will allow you off the Q400 is by waiving the contractual provision that freezes you in type until upgrade OR through displacement. You can NOT bit on a vacancy position as an FO unless it's the same type and certificate that you are currently working. Initially, I expect the Q400 FOs based in the east bases (EWR, PIT) to accept voluntary displacement to both YX and S5, the Q400s FOs based in DEN and MCI will probably stick around unless they allow a displacement to the YX MCI E170 base.

So in short, there will be displacements from the Q400 FO side, but based on your seniority (and how others bid), you could end up at YX or S5. There is really no control and it will amount to luck if you end up getting displaced to a certificate you want to be on.