Copy FG Tile map

When making levels, I first go into Photoshop and make a rough outline. Then I paste it into MASCOT and make massive changes adding rooms, changing many tiles. I am not quite sure how to paste xichrons. If my level looks completely different from the orginal photoshop image, how am I to make the xichrons? Also if I paste some xichrons on will my old ones dissappear? Is there a way to copy tile map, paste it in Photoshop and see where the xichrons are to be added? Thanks in advance.

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