That is not the definition of 'dictatorship' generally used in the social sciences. Normally it is defined as control by a person or small group without any serious constitutional oversight. It usually means there is no independent judicial branch. Another problem with your approach is that it completely ignores the other branch in modern democracies, that of the legislative branch. The whole principle of separation of powers is non-existent in dictatorships. Since neither the legislative branch, nor the judicial branch are independent there are no real institutions that can check if the rulers are abiding by any kind of constitution. Same goes for the press, which is usually very restricted in dictatorships.
Some definitions of dictatorship:
http://wordnet.princeton.edu/perl/webwn?s=dictatorship
http://www.britannica.com/eb/article-9030347/dictatorship
http://en.wikipedia.org/wiki/Dictatorship
Thats why I said the process of constitutional change is lengthy and complex. In almost all countries it requires at the very minimum fresh elections and thus a fresh parliament. Any rulers wanting to change the constitution will have to face the electorate before their changes can become law. So they are very much restricted in their actions. Formally, a ruler can't grant himself more powers based on a one-time popular mandate alone without first facing the 'will of the people'.
It is not correct though. There is no opposition in a dictatorship, there is no independent judiciary, there is no free press, no right to demonstrate, no right to create political parties, there is no independent legislative branch. All these factors, which are as much considered a core of modern liberal democracy as elections, fundamentally change the dynamics of rule. Dictators are simply not restricted in such ways.