Copy/paste new images onto the text image
Make sure the text is sized and placed where desired. Once layer masks are created, the text can't be edited, so it's a good idea to lock the text layer.
Use the text layer to APPLY IMAGE to a layer mask on one of the added image layers and INVERT the mask.
Clip all of the image layers to the text layer.
Duplicate the mask for each of the other image layers, then fill all the mask with black except the desired letter.
Unlink the mask from the image in each layer. Then each image can be moved and transformed to fit within the letter.