Is there a separate character for the flashback?
You don’t need to create more than one character. You can use your main character and if you want to change the appearance of that character to make them younger, etc. you would just use the previews command to change their look. Then to change the character back to how the readers CC’d them, use the unpreviews command.
For example, if you want the character to look younger for a flash back scene and want to change the hairstyle and face shape, you would do this:
@CHAR previews face Square Youthful
@CHAR previews hair Full Pigtail
Then after the flashback scene when you want to revert the character back to the original look:
@CHAR unpreviews face Square Youthful
@CHAR unpreviews hair Full Pigtail