Upgrade form Tourer to Tourer+

I bought the Kurviger subscription Tourer for €14.99 today. Now I realized that I need the Tourer+, which currently has a discount of 33% (19.99 €, instead of 29.99 €).
When I select “Change subscription”, I only get the option for 29.99 €, not the advertised €19.99 variant. Do I have to cancel first and then buy again via the google play store, or can you help me?

If I need to cancel first, what is the easiest and fastest way?

Are you on the website or app?

I bought the app from google play store and wanted to upgrade from within the app.

Edit: When I try to upgrade through the website, I get a notice that it can only be done through the app because that’s where I made the original purchase.

Ok, so there are a few things that you can try, the easiest would be to just restart your phone. If this does not help, please have a look here: Keep your device & apps working with Google Play Services - Google Play Help

It could be that you need to clear some caches of Google Play if you can’t sign up for the offer. If this does not work, please let us know.

Unfortunately, your tip didn’t help.
I have now cancelled the trial version and subsequently purchased the Tourer+ with the 33% discount.
Thanks for your quick response and support.