I'll have to disagree Av8. Speed, Mach tuch/buffet etc.. are a few issues. AND not just TBP to Jets but Narrowbody to Widebody and Classic to EFIS.
Now this example is from years ago and I know does not always apply however; My father went Left Seat L188 to Left seat DC8 but had previous DC8 right seat time. A lot of guys did NOT make it! Also, to move up to the widebody you needed time in type so although he could hold the DC10 and 747 he had to fly right seat in it first.
I would think that insurance, and the major airlines decisions, might have more to do with it.
I am not sure what Piedmont is doing.