A higher-commission link changes what your next video earns, not what your old ones do.
Why old videos keep the old rate
Videos and Lives published before you updated the link track the rate of the link they were posted with, and that includes a viral video that's still generating sales today. If that link was a TAP link, its rate lasts until the campaign ends. The link a video was posted with decides its rate, not the link currently in your showcase.
When the higher rate kicks in
The higher rate applies to new TikTok Shop videos and new Live sessions created after the higher-commission link is already in your showcase. Post after the link is in place, and that content tracks the new rate from its first sale.
Next steps
If the rate itself looks wrong, start here: