Assuming you start with the nitro to try it out, carb it with straight CO2 first anyways, just to save the nitro. Except carb it to lower levels than you normally would, maybe 5-7psi (1.1-1.3vol) or so, depending on your setup. Then you can switch it over to nitro at around 30psi or so, which should keep it near the same CO2 pressure or close to it since the ratio of nitro to CO2 in blends varies a little bit. If you decide to take it off nitro, just vent the keg and the beer will still be carbed to the low level (1.3-1.5vol), and then you'd just have to bump it back up a little higher with CO2 to get to a more standard carb level. Although after you try nitro, I'm not sure you'll want to go back!