Does learning new positions really eat into CA?

I want to convert Pastore to an inside forward but I'm not doing it if it will stunt his growth

It won't, but keep in mind that training time is taken up by learning a new position.

I want to convert Pastore to an inside forward but I'm not doing it if it will stunt his growth

Not directly but indirectly it does.

More positions = more attributes being heavily weighted = more CA being used up.

The exception to this is when you train from one side to the other as the attributes used are the same.

'jack of all trades, master of none' is a statement that probably rings true. If your player spends time learning multple positions then this will mean less time and ability to devote to getting really good at something else.

'jack of all trades, master of none' is a statement that probably rings true.

Definitely agree. Only retrain to another position if it is essential

But it also means some heavily weighted attributes are then less heavily weighted and more balanced out, like how at least in FM10 you could get your striker even better by training him to also be a sideback, reducing the CA cost for useful striker attributes.

Jack of all trades master of none doesn't really apply to a pro footballer in this instance.

You are just shifting the weight of attributes slightly differently. Attackers who work hard and tackle aren't considered poor, they are considered great - like Tevez and Bellamy.

Defenders that can pass, shoot, dribble aren't 'poor defenders' they're Bale, Maicon, Dani Alves.