Welcome, my name is XXXXX XXXXX I will do my best to help you with your issueMost of the time these units are 1 piece or essentially 1 piece. What you can do is install a replacement window inside the existing casement window frames after removing the sash
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OK, so it is possible to leave the fixed picture window in place and replace the two on the sides?
Yes you just remove the casement sashes and insert the replacement window it their place. The sash is the movable part of the window