Scheduling more than 30 days out is purely notional these days. Not just for B6, but most carriers. The issue is aircraft utilization. Whenever OP scheduled, the most efficient use of the aircraft in question was likely the AM departure from the East Coast with ground time in SEA before a red eye back East. Now, presumably there is a more efficient use of that aircraft and that is what B6 has done.
The only reason anyone would even know that there has been a change is that B6 las limited Mint-configured aircraft and thus a determination has presumably been made that the swap is more efficient.
Given that OP is not flying until June, this may well change again.