I'm considering making a deal for a young pitcher aged 20. The previous manager has him in the majors even though this is about to be his third season. What's the best move for his growth? Should I keep him in the majors so he doesn't get frustrated? Or since he's changing teams, can he be dropped to AA or AAA without him taking a development hit? Thanks

You can send him to AA/AAA.     But, if he's BL-quailty, there's no harm, development-wise, in keeping him in the bigs.

You *do* risk a demotion penalty (not sure if that is what you mean by development hit) if you assign him to a level lower than where he is now, but it is not guaranteed by any means.

Keeping him in the ML won't stunt his development if he pitches as regularly as he would in the minors, but you are burning ML service years, so the financial impacts are worth considering indepently of the development.

Unless you've actually gotten a demotion hit when assigning a player, I'll question this "You *do* risk a demotion penalty".   I've done it dozens of times, if not more, and never received a hit.

I have actually received a demotion penalty from assigning a pitching prospect to a lower minor league level than the one he was at for his prior team. I later confirmed via ticket to CS that this was working as it was intended. The relevant (to this discussion) portion of the response was - "Additionally, if you're assigning a player to a lower level then he was playing at there is a good chance for a demotion penalty..."

It's happened to me, also.  Not the majority of times, but it does happen.

I guess I've been lucky many, many times. 

i have also been lucky every time...ever