My BOS flight on 4th May is showing still at £300. If it dropped to £250 or less id take it.
In regards to your AC swap. Most likely due to the fact the new Neo G-VSRB was supposed to be in operation from 6th May, and now it’s only expected from 20th May. On May 4th they scheduled 3x JFK and 1x BOS so barring any mechanic problems, should be ok.
We had the same situation last year with our Tampa flight where they scheduled 3 Neos as they were expecting the 3rd to be delivered but it was late delivered; thankfully they swapped the AC on the other flight and kept Neo on our flight.
See below current schedule of Neo operations, subject to change due to late deliveries:
May 20th:
5 Neo: 1x TPA, 4x JFK
August 8th:
6 Neo: 4xJFK, 2xBOS
5tH September -
7 Neo: 2x BOS, 1x TPA, 4x JFK
Sunday 27th October
1x IAD, 1xMIA, 4xJFK, 1xTPA, 1xBOS