The usage of "conversion"

In the Oxford Learner's Dictionaries, conversion means "the act or process of changing something from one form, use or system to another."

How to understand this "something"? If someone changed his behaviour, is it valid to say "the conversion of his act surprised me"?

As a native speaker of English, if I heard someone say "the conversion of his act surprised me" I would be confused, so I wouldn't recommend using "conversion" like this. Conversion when applied to people tends to have a religious meaning, e.g. he converted to Roman Catholicism.

I also have a problem with "act" in this sentence because an "act" is a one time event but "behavior" is an ongoing pattern. If you're talking about someone's behavior you can't use act as a synonym.

It isn't common in everyday English speech to use conversion as a synonym for change in the sense of "difference." For example, we would be far less likely to say

Everyone noticed the conversion in his behavior after the incident on the freeway.

than to say

Everyone noticed the change in his behavior after the incident on the freeway.

or

Everyone noticed the difference in his behavior after the incident on the freeway.

or perhaps

Everyone noticed the alteration in his behavior after the incident on the freeway.

Conversion is much more likely to be used in connection with the event of switching from one state of action or belief to another than in connection with the resulting difference in behavior. So we might use conversion in a situation like this one:

After his conversion to an alcohol-free lifestyle, we noticed a change [not "a conversion"] in his attitude toward school.