I don't know the answer because I've never used the tool.
But you don't have to redraw everything. You can use the copy tool to copy 1 row (or column) at a time (hopefully you aren't on a 150x map requiring 150 copies!). Eg you want to do a Left-Right mirror map on a 50x map and you did the first 25 columns. Copy column 25 and paste it in column 26, column 24 in 27 and so on until you copy 1 into 50. Tedious but not as tedious as starting from scratch.
You'd then have to manually add sites/cities since the copy doesn't copy those items.