change alignment

Can you change your character's alignment in the middle of a campaign? I'm thinking of making a fallen aasimar paladin that will eventually become an oathbreaker and I want to know if I can change its alignment from lawful good to chaotic good when it becomes an oathbreaker.

If your character's personality changes drastically, than yes. I personally don't use alignment and use alignment specific things by how the player RPs.

Make sure your DM is ok with it. Also, check with the rest of the players. Many people do not want to be in a campaign with evil characters. 

I also want to know if paladins are even allowed to be any alignment other than lawful good. I heard somewhere that paladins have to be lawful good.

I wish there was a less evil oathbreaker. How come as soon as a paladin decides that they don't want to serve a king/God anymore, they automatically get necromantic powers?

Anyway, yeah alignments can change. It should either happen slowly over time or after a major character development moment.