I think left-handed conductors, mostly, beat time with their right arms, and it is not very difficult to learn to switch arms. If I injured myself, for example, I think in about a day I could switch arms. It would be a little confusing for the orchestra, but the problem would quickly be surmounted. I knew of one conductor who lost the use of his right arm and used his left. His big challenge was memorizing more since he could not turn pages readily.