I got you now, but it's still not a good idea. As you start planing, you will be planing uphill against the grain on the curved section and there is a likelihood that it will tear out. If the cut is deep enough it could be nasty.
You could start in the middle and work towards each end, but I'm not not keen on doing that either.
If it was me, I would cut out the curve and sand it, then plane the sides square to it.
Dave.