Well Hitler only had the opportunity to change the constitution through the Reichstag Fire. If it doesn't happen and Hitler takes a moderate and cautious line then it's likely that he'll be replaced by the conservative elites as the economy recovers and support for the NSDAP decreases. Within the NSDAP itself, the failure of Hitler's legalist line would probably result in a putschist takeover of the party, possibly by Rohm. This would probably lead to a SA coup which is crushed by the Reichswehr. So overall, I think that there would be a conservative military dictatorship in the long run, without the presence of the NSDAP.