I have a question about the credit card and pricing update

It has been my understanding the when prices increase, existing sites do not increase and as long as they do not cancel they keep their original pricing. I just created a price update. I have an existing site needing to change the credit card on file (stripe) but she is seeing the new pricing, not her existing pricing. Please clarify what SHOULD be functionally happening.
The user currently has a site with the old pricing. However, there is a problem with the credit card and she needs to change the credit card on file. I was told the process would be to go to the Account page and select plan and the check-out appears. However, only the new plan pricing is showing. So I am wondering will the credit card change create a new subscription with new pricing or will it actually only update the credit card and the old pricing will continue?