Custom pricestring is replacing the "upgrade" string when switching

[bernattorras]

Description

When you switch to a product that has a custom price string and a sign-up fee needs to be added to the cart as a proration, the product custom price string overwrites the sign-up fee string that is automatically added from the switch process.

Steps to replicate

1- Create a variable product
2- Set custom price strings for each variation
3- Enable subscription switching and make sure you "prorate the recurring payment"
4- Purchase the cheapest variation
5- Switch to the more expensive variation

Expected result:

The product price string should include the custom price string + "and a $X sign-up fee (Upgrade)"

Result:

Only the variation custom price string is appearing, overwriting the upgrade addition.