Since May 2019 we have switched to VISA. You can still use your Mastercard as usual, it will not lose its validity in the future. Nothing else will change either.
If you want to exchange your Mastercard for a Visa-Card, please check your address in the app. If this is up to date, you can simply order a new one. This can be done under "Account" → "Card" for free.
With the change to Visa we only change the card provider. If you have any questions about the Impact and specifically the climate protection contribution, have a look at the FAQ's on the topic "About us".