Are you just wanting to use the one character for the present as well as flashbacks or did you want to use 2 different characters — a younger one and the present one?
If you want to use the one character, I recommend using the preview commands e.g:
@CHAR previews hair Straight
@CHAR unpreviews hair Straight
When you unpreview a feature, they go back to whatever it was before, so it’s generally used for temporary features without you needing to remember what readers’ selected.
If you want to use 2 different characters, then I recommend the becomes command e.g:
@YOUNGERCHAR becomes MC
@YOUNGERCHAR changes face into Square Youthful
@YOUNGCHAR changes hair into Pigtails
@YOUNGCHAR changes into YOUNG_OUTFIT
The becomes command allows the younger character to literally become a copy of the CCed MC, and then from there you can change a few features to suit your vision, like a younger face shape or “kid” hairstyle etc. it’s best if you get readers to select a natural lip color though.