I think you'll find the change of flight number coincides with a change of departure time that will be incorporated into the new summer schedule (hence the date)
This is much more to do with reassuring passengers than outwitting terrorists.
The sales on this flight have suffered (predictably) while all other flights to Washington have continued to sell well
There has been a lot of discussion as to the relevance of the flight number, but I'm sure that this decision has been undertaken after careful discussion with intelligence departments.
The fact that other Washington flights have been unaffected demonstrates why the flight number being changed along with a schedule change should resolve this issue.