Well, when you create a warlock you have to chose your patron at the start so fiend and hexblade are exclusive because they are different subclasses.
I guess Wyll will always be a fiend warlock because it makes sense for his story.
If you play with your friends and you can convince the GM that patron xxx can give you the powers of a hexblade, thats a different story.

Prof. Dr. Dr. Mad S. Tist

World leading expert of artificial stupidity.
Because there are too many people who work on artificial intelligence already