Gear position - Neutral or Parking while doing Transmission Fluid change

It doesn't matter...... I've done mine four times......you can't get in Neutral when the engine is off.
Use a funnel and at least a three foot hose. Keep pouring until it comes out the top hole. Remove top bolt first. That way if for some reason you can't get the top one out your not out of fluid and can't put any in.
Use a hose small enough to go inside the trani a inch or two and not real tight so the fluid can come out when it's full.

 

P⃞ and N aren’t different gear positions. In the P410 transaxle, the gear train is always in mesh, and there’s no shifting of gears in the usual sense. Rather, the shift states (D, R, and N or P⃞) are just different ways, electrically, of operating the two motor generators. The only mechanical difference between P⃞ and N is whether the parking lock mechanism (parking pawl) is engaged.

I doubt it. The Repair Manual (more info) has two procedures for the hybrid transaxle fluid: “Replacement,” to drain, refill, and run the engine in inspection mode, do that again, and then drain and refill; and “Adjustment,” to drain and refill only once. If you want all of the old fluid out, do the “Replacement” procedure—but you’ll need enough new ATF WS to refill the transaxle three times.

It’s not necessary in this case, but you can turn a Prius car off in N. Toyota’s recommended procedure (see the Job Aid in the Course 072 handbook, available by subscription to techinfo.toyota.com) is to pull the P.CON.MTR relay before shutting down the car, which prevents the parking lock from engaging. It also sets a diagnostic trouble code that has to be cleared later.

 

Just drain, with the car off, refill with car level, and when it starts coming back out level is good.