In Star Wars Episode 9: The Rise of Skywalker Rey can hear the voices of several dead Jedi at once, which includes Obi-Wan Kenobi voiced by both Ewan Mcgregor and Alec Guinness; this means that she is hearing the same character but 2 different voices. Why would he have 2 different voices, especially considering that every other appearance as a Force Ghost he was his Alec Guinness self (citation needed)?
I don't know that there's an in-universe explanation for that one. Out of universe, ROJ is filled with lots of subtle tributes. The Ewan McGregor line is from The Force Awakens
“I don’t remember where I speak, no idea,” McGregor admitted with a laugh. “I think the beginning of the line is… the character’s name is Rey. I’m really struggling here. So they wanted a line of dialogue that said ‘Rey…'”
Obi-Wan’s full line in the film is “Rey… these are your first steps,” which Norton tells him after watching the actor struggle. In McGregor’s defense, he is only in the film “for a second,” as he says, and his cameo was a pretty last-minute addition.
JJ Abrams said he wanted to unite the trilogies
[Episode IX] is also the film which unites all three trilogies and brings everything together.
Granted, the scene continuity doesn't make as much sense with two different eras of Obi-Wan, but including Alec Guiness in a Jedi montage is a simple way to include the original actor, as well as the new one.