I want to buy the PRO licence. I will in the future need the Developer one, but for now I only want the Personal one. If I buy the Personal one, and then in the future switch to a Developer one, will I have to pay $100 or will I have to pay the difference of $75?
You will only pay $75 AUD for the upgrade.
This can be done from your store account by following the prompts on the “Upgrade license” section.