There is a difference in REPLACING vs ADDING sprites.
When ADDING sprites you are adding a new "set" of sprites. Back in Stage Mode you must use "Sprite Switching" via the Sprite Editor to change to those sprites. This is similar to changing to different hand shapes. Once changed you can save your character with those sprites showing.
When REPLACING the existing sprites you will see those sprites when you return to Stage Mode.
When returning to the stage from Composer Mode the character will be switched back to the default sprites that were originally selected.
If you had Replaced the default sprites with new ones they will show up.
If you Added new sprites you have to switch to those new sprites to get them to show.
See my CTA Tutorials on the YouTube channel CTAStepByStep
Check out EZColors for CTA. The easy way to change Render Styled (RS) colors.