I’d be curious to see a rough idea of that.

It may be possible to enlist the help of those who worked on TFA: Restructured who altered Finn’s line at Maz’s castle. It was done perfectly. Granted it was a lot of work and trial and error, but the results speaks for themselves. Lookint to Obi-Wans scene, if it helps, a few words/mouth movements that are needed for Obi’s lines to help that scene be seamless, that would pay off! Words and sounds could be used for that editing which can be found throughout that movie, along with content from Revenge of the Sith to use. Even other movies if the right sounds are not obtainable, though that can be a bit tough to have the sound of his voice match.

Here is another example of the same technique to help a possible continuity issue:

OBJ: Continuity > ITEM: Obi Wan’s instructor > MEDIA: EpV/EpI > ISSUE: Obi Wan in EpV says Yoda trains him; quote: " There you will learn from Yoda, the Jedi Master who instructed me. > NOTES: Qui Gon Jinn was his DEPICTED master/instructor > POSSIBLE SOLUTION: Change Obi Wan’s line to “…learn from Yoda, a Jedi Master who instructed me.” This helps open up that Obi Wan learned under multiple Jedi Masters. From prequels, there were plenty of masters who could have trained/instructed/taught different aspects of the force their were more “natural” at … be it meditation, light saber techniques, the living force, etc. With the proper mouth manipulation it can be done seamless since there is plenty of resources where he does make the A/Uh sound.

Granted this is a minor one which can be simply interpreted that Obi Wan was not going to bore Luke with the details. He hasn’t really been completely honest in all things with Luke anyway as well. Shruggy Thing