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But, yeah, these RP to SP conversions definitely follow that pattern.
Mostly starting in the minors
Introduced as 80% RP's, or 100% RP's, in the majors
Tried a few times in the rotation, failed the first time or two
Changed to the rotation later, big success.
Would guess that you're right -- that pure relievers out of college and the minors are seldom converted.
Wilhelmsen in the minor leagues has had 41 starts and 5 relief appearances to date.