1
For a time, the Twins were masters of taking a starter and "introducing" him to the Majors as a reliever.
Johan Santana
Francisco Liriano
They both began their MLB careers in the pen. But, they were always SPs in the minors. Not sure how you quantify the type. Does Morrow count?
I would suspect very few RP to SP conversions were attempted with guys who came up as relievers in the minors. I suspect most were always SPs and 'snuck' onto rosters in the bullpen due to situational circumstance. TW may qualify there ... but really doesn't have enough of a minor league resume to draw too much in the way of conclusions.