It’s hard for many players and parents to understand but most coaches want to win just as much as you job, and their job actually depends on winning.
A position move may be necessary for a few reasons. There may have been an injury, suspension or benching that leaves a deficiency at another position, and moving you to fill that gap may be a better option than bumping up the backup. Your backup may be a better backup than the backup at the other position. Or, by moving you to another position and moving up your backup, your coach may feel he has an overall stronger roster.
Coaches usually make decisions based on what is best for their team. Not what is necessarily best for each individual player.
Position changes can play out in your best interest, and the transition is easiest when you don’t fight the decision and instead focus on becoming a master of the fundamentals of your new position. Take the new challenge on with a great attitude, and see where the move takes you!