when you mount with the ears forward you leave much more of the ear butt in so to move it ears back you would have to cut stitches in hide and if hide was not flexable enough (which depends on the tanning method) rehydrate the hide to a point that it is flexable, then you would have to remove more of the ear butt and reset new clay with the ear liners.

Worst case senario you would have to remount the deer with new cape. Best case is the cape is flexable enough or could be made to be and you would be able to just pull it back enough to rework the ears.

Good Luck, I would suggest next time to ask to see pics of ears both ways so it is easier to decide. You are right to catch it now where there may be a better chance to change it than later. But it will likely be a major operation.