Not sure if I'm just missing the option here, but I cannot find the pay button in my-account to upgrade from the core bundle to the developer bundle, I have searched and there is a blog article from 2017 stating that the pay button should be within my-account.
We have resolved this issue via email. I'm posting it here in case anyone need a reference:
We’ve switched to another payment gateway which now performs all the payments and subscriptions on the website. We’re unable to do upgrade with it anymore. If you bought the Core Bundle within 14 days, we’re happy to refund and then you can buy the Developer Bundle again.