This is what the current CCAR121 says:
Article 121.417 pilot initial/transition/upgrade training entry requirements
(a) (2)for airplanes with max takeoff weight greater than 136,000kg(exclusive), the pilot must have been captain on class II airplanes for more than one year and have at least 500 hours captain time and total flight hours must be no less than 4000 hours; if the pilot does not have the above mentioned class II airplane captain experience, his/her total flight hours must be no less than 5500 hours, including no less than 2500 hours on class II airplanes, no less than 450 legs including takeoff and landing as PF, among which no less than 250 legs including takeoff and landing as PF on the said type.
So in other words...legally at least I understand that an FO without previous PIC time in a medium/heavy acft and holding the above mentioned minimuns can be upgraded to CPT directly to an WB acft. However, I understand that in practice this may as well be another story...any thoughts?