Hello
Bean
I went through this exact process in BA during the summer of 1998. Dates are from my log book so you can quote this back to de-bunk their nonsense.
Last MAN 737 flight on 2nd May '98 and first line training LHR 757 sector on 19th June '98 thus the whole of the conversion - groundschool/fixed base, exam and 12 sim sessions took 48 days or a month and a half.
The fleet rule was 6 months on line only rated on 757 so my differences course was a few days over the second week of November '98 with a single sim session on 15th Nov and a check flight to Rome the next day.
So the conversion 75 to 76 was 8 days tops!
As an aside, the next time I touched a 76 was exactly a month later which did have me looking at my notes
