I have building out a wall in a bedroom to make a larger closet. The return is currently in the wall part of the wall that I need to tear out. Is it suitable to use flex duct to relocate the return to a different location?

Yes you can. If you need help determining the correct size, give me the ID of the return

Hi rodri. I don't have much HVAC knowledge. What exactly do you mean by the ID of the return?

Inside diameter of the return opening, or look at the size of the supply and multiply it by 1.5 and that will be the size of the flex for the return