Listen to your coach. He's trying to make you the best pitcher you can be. If you're both successful at that task you won't need to worry about the outfield.

Side arm may not work in all situations you should learn to throw overhand any way. :)

I think it is a good idea because when you throw side arm you sort of sling it.

This is going to be a different answer then what you're probably used to and it will change everything.

Throwing from the outfield and throwing from the mound are two completely different tasks. If you really want to further your pitching and protect the arm, consider informing your coach to leave the arm the way it is naturally. A lot of coaches want to make the adjustment because the feel that it will help. It's just bad information as time goes. They are still great coaches, it's just bad information.

Due to way that you move and muscle tension in certain areas, there's a certain way you throw and adjusting that can force tension and stress to certain areas that promote injury.

Plus over 80% of rotator cuff injuries are from overhand overuse, not sidearm. Just make sure that your elbows are lined up with your shoulders and that's a start in a better direction.

Throwing from the outfield is a different story. Throwing sidearm from that distance can be pretty tough. Your throwing mechanics will play a huge factor. Throwing overhand would benefit you more from that position. I agree with Gabri (sidearm does not work in all situations). Just figure out what you want to do more of and where you're passion is.

There's more into it but when we have a pitcher running through our pitcher's program we don't force them to go from sidearm to overhand, it will actually slow down arm speed because of the un-natural change.

If you have any questions or need further help let me know.

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Coach Beard is right but I would be very careful with the way you inform your current coach that he "may" be wrong. (I say "May" because he might have picked up on a mechanical issue that may be detrimental to the health of your arm. Throwing sidearm may relieve some pressure in your shoulder or rotator cuff but it also tends to add more stress to the elbow.) As you get older, pitching coaches usually will leave your arm slot alone and focus more on the mechanics that get you to the release and finish of a pitch. As an outfielder you will have to adjust your throwing mechanics so that you do not get the tail on the ball associated with throwing sidearm.

Greg is correct. You wouldn't want to inform your coach that he may be wrong. Asking him to keep your arm in the natural movements will help a lot in "preventing future stress", which most find hard to pay attention to because people want to make changes now and see a result now (not in all cases). Reminder that he isn't wrong- there's just a better way with the arm and that's not only being patient, also with the mechanics (which Greg is right on about). Most of our stresses come from bringing things out from our natural movements whether it's good information or bad.

Great additive by Greg about the elbow stress.

Elbow stress most of the time will be created from muscle tension and imbalances. Muscles can through some imbalances whether sidearm or overhand and most of the time it's with side-armers because they use more of certain other muscles that overhand throwers don't and vice-versa. Which can be relieved with the proper attention (the good information). So if and when that happens see you're local sports massage therapist to help relieve that stress, drink lots of water because hydration play a big part as well as some good old rest.