What is the difference between 变 and 变得? What is the difference between 变 and 变得?
Is 变得 a word in its own right, or is 得 some sort of complement word?
For example, 眼睛变得亮晶晶。
得 is some sort of complement word here. It tends to be some sort of comment or description of the action in the verb.
The basic structure is:
[verb] 得 [description]

From your example

眼睛变得亮晶晶

变 is verb, 亮晶晶 is description of the eye.