You could have him sound like Jason Wingreen and explain it away as either a voice modulator upgrade or a mysterious man with no name has killed PT Boba and is trading on and advancing his reputation as a badass bounty hunter.

I never saw any actual character rivalry between Fett and Solo.

Solo knew that Fett was the best at what he did so Jabba would eventually hire him.

Fett takes jobs seriously.

That's the limit of their actual in narrative enmity so far.

Fett does represent a mirror of Solo. He is an outlaw, he has the cool look and the fast ship but no rebel friends. In fact he is more solo than Solo which serves as a comparison to Han's character arc.

I suspect if they knew each other personally they would probably get along reasonably well.