Mixing PVC Conduit Schedules
You can mix them, as long as the places where you have schedule 40 elbows are places where schedule 40 is permitted.  

It never came up but I'd be surprised if you had any issues.  You ought to make sure that the fittings match, for example if you use a coupling between the schedule 80 and the schedule 40 elbows, it must be rated for both types.  Similarly, if you use the bell ends built into the conduit or elbows you would want to check with the manufacturer to make sure it's listed for mating with the other type.