Although the short and long-haul changeover actually took place in late-March 1978
That would make sense - so the flight numbers were rationalised at the end of the Winter 1977 IATA season, and the BE prefix (and Bealine callsign) finally disappeared at the end of S78.
It would have been very difficult to do either of those mid-season, I have a vague memory that's roughly when the res systems moved from BABS and BEACON to RTA, but I may be wrong about that.