I know the dictionary form ends with う、す、る、etc. But what about the changed forms of them such as 買った、食べなかった? Can I also call these dictionary form?
Dictionary form is exactly that: it is the form by which you can look up a word in a dictionary.
It you take a verb like 買う you can inflect it to get other forms such as 買わない or 買わなかった or 買った. But, try as you might, you will not be able to look up these forms in a standard dictionary. You need to be able to recognize that they all derive from the dictionary form.
Typically, for someone learning Japanese, you should note that these forms are all informal forms. Their formal counterparts being 買いません, 買いませんでした, and 買いました respectively.
Generally, the informal forms are found embedded in a sentence. For example,
The guy who bought the book is over there.
The dictionary form is often the starting point from which you derive all else.
If you want a name for the other forms, I would suggest calling them either derived or inflected forms. Most people proficient in Japanese will understand what you're referring to.
I would say that plain and dictionary forms generally refer to the same thing. I would say that casual and informal also refer to the same thing. But while all plain forms are also informal, the same is not true the other way.
If you need a catch-all to describe the verb forms you are talking about, I would say something like "the dictionary form and its other inflected informal forms". That's a bit of a mouthful. But, I'm not sure that you would need to talk about these all together very much (unless you intend to become a grammatologist--did I just make up a word?).