Narnia actors that could double for different roles? Only a few. As long as it helped the story and didn’t mess with continuity. Here are some examples at the top of my head:

Young CASPIAN actor from PC = Young Rilian actor  from SC (different hair color).

 - I’m going to assume that The Lady from SC is a shapeshifter of sorts. - 

Ranandu’s daughter Actress from VDT = younger Green lady actress in the garden from SC (different hair color). Yeah, creepy.

A Nyad actress from PC = The Queen of Underland actress from SC (if the Lady is a corrupt nyad).

Older Rilian Actor = Adult Tirian actor (different hair & beard).

Aslan = the Lamb.

Weird examples, but there it is. Thanks!

In a way, if an actor played more roles in a production, it would probably decrease the link between those characters in an audience's mind. A long time ago, a friend of mine wrote a play which he performed at the Greenbelt Festival in England where he played all the male characters and a young woman played all the female ones, and that was the premise of the play - people didn't think that all the characters were connected because they were played by the same two people.

Posted by: @jfg-ii

Young CASPIAN actor from PC = Young Rilian actor  from SC (different hair color).

Posted by: @jfg-ii

Older Rilian Actor = Adult Tirian actor (different hair & beard).

I think this is kind of a cool idea since the characters are supposed to be descendants. But then it'd be harder to have the big reunion of all the heroes at the end of The Last Battle.

Posted by: @col-klink

I think this is kind of a cool idea since the characters are supposed to be descendants. But then it'd be harder to have the big reunion of all the heroes at the end of The Last Battle.

I get it. In a perfect 'JFG II world' - where all Narnia movie were released two years apart from one other - 20-year-old "Young Prince Rilian" could play 26-year-old King Tirian three movies later. Ditto.

P.S. I always thought that actor Ben Barnes was much more suited to 26-year-old Tirian than teenage/young adult Caspian - even 30-year-old "Black Knight Rilian" suited him more.