Adding new joists to old deck: lumber size mismatch
First make sure that your support posts and beam(s) are in good shape.  You'll be adding a lot of weight doubling the number of joists and adding the heavier  PVC decking.

Your choices are to either allow the new joists to hang lower than the others, or if you don't like how it looks, cut a 1/4 inch off the width before they are hung.

No matter which choice you make, the joints are still hung flush on top.