yii1 to yii2 code changes

I found an amazing extension to add to my project, however it is only in yii1.

How hard /intensive is it to migrate that code to yii2

(I am a beginner to yii2, and unfamilliar to yii1 at the moment)

Would that need a drastic recoding of things, or would it be fairly simple?

It would need a "drastic recording of things", i.e. complete rewrite and retest.