Prob on the next Vacancy bid. Those happen about every 4-6 weeks and lately the 787 FO slots have gone very junior with some going unfilled (thus the assignments to new hires).

Not recommending that you take a 787 bid immediately, but taking a narrowbody assignment does leave you open to take a 756/777/787 whenever you desire as you wouldn't be frozen from bidding them. First year pay is the same regardless of aircraft assignment, so you would only see a financial benefit starting in year 2.