it is normal to get more rpm with the AUTOPROP, as this is a self-adjustable pitch propeller with free rotating blades.
When the vessel has no speed, you can really get full power (3400 rpm) from the engine as the blades have very little resistance. As soon as the vessel gets some speed, the pitch changes because of the water flow (making the blades turn a bit) and then your rpm go down.
With a fix prop, the pitch does not change, and the propeller resists more to the engine torque. As the pitch is fixed, you will probably get a maximum of 2800/2900 rpm. This is normal.
In my opinion, an AUTOPROP in good condition is better than a fixed prop. The idea of supplying a fixed prop in every SM was just to give a back-up possibility in case of damage (or maintenance) of the AUTOPROP.
Could you tell us why you changed your prop?
Have a good day.