I poked at my twitter account, and I figured out how to pay for twitter blue and change my username only once after paying. Here are the steps that worked for me:
1. I paid for Twitter months ago but let it lapse, so it shows up in my previous iCloud purchases.
2. You can buy Twitter blue from that screen for $7.99
3. If you convert your account to a pro/creator one, you are allowed to change your username ONCE so be ready
4. Wreak havoc