Some additional information for anyone else who may be encountering this issue:
I have contacted support for this, and have not had a good experience. I thought that they would change the card denomination to make them usable by anyone in my country, but instead they have applied all my gift cards (which I intended to give to friends) to my own account. To anyone else contacting support, be very specific about what you expect them to do.
Digital Robux codes that redeem directly to Robux (such as the codes sold on Amazon or obtained in programs like the Microsoft Rewards program) will continue to be redeemable globally.
However, on Amazon, the purchase of these codes is restricted to customers located in the US.
Digital and Physical Gift Cards that redeem for Roblox Credit (like the majority of cards sold at retailers like Walmart, Target, etc) are now only redeemable by users when their country matches the currency of the gift card.
This is the type of gift card available from the front page of the Roblox site, which they are continuing to sell to international customers who will not be able to use them. This seems wrong to me. Everyone, please be careful.