Leave Temuera Morrison as the voice of Boba Fett, but maybe tweak the tone or speed to make him sound more like the older, menacing Fett voice.
Also have someone speaking out loud Fett’s name in Ep V.
Make Obi-Wan call Darth Vader “Anakin” or “Vader” instead of “Darth”.
Trim Leia addressing Obi-Wan as Ben.
Add at least one reference to Vader and/or the Emperor being a Sith (for example, in the Ep V opening crawl, introduce Vader as a “Sith Lord” instead of “evil lord”.
Address the Emperor as “Palpatine” in at least one line (it could be during the Rebel briefing scene in Ep VI: “…we’ve learned that Emperor Palpatine himself”).
In Ep VI, alter Luke’s line, from “I can turn him back to the good side” to I can save him. I can turn him back to the Light side".
Maybe one reference to the balance of the Force?
Maybe a throwaway line confirming Bail Organa’s death?
Add movement and Aurebesh texts to the screens of the Rebel bases and Imperial systems.

Also there might be some ways to further bridge Rogue One and the OT.