I don't think it's entirely necessary to replace the chain at the same time, yes it's a good idea so that they wear evenly (both sprockets and the chain), but if your chain and rear sprocket are in good shape you should be fine. Just remember to keep an eye on the other stuff for wear since they will need replacing before the front does again.
BTW when you're replacing that front sprocket, it would be a good idea to add a case saver, it's cheap insurance that IF your chain skips or snaps because it's worn, that it won't bust your case. You might also consider cutting a couple of the "fins" off the stator cover so you can get a socket on the front sprocket nut to change it easier, that way you don't hafta pull the footpeg, shifter, and stator cover just to swap front sprockets...