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

I simply changed my username from Matt Haughey to Matt after getting errors trying to change my username before. This looks like a loophole they forgot to close, and if I had all my twitter impersonation jokes ready to go, I could have pulled it off for 8 bucks.
@mathowie So they’re just checking for… a substring match?
@kathrynyu I think they block all display name changes right now, but when I converted to a pro account, it gave me one chance to change it before being blocked again. I think that's the hole that can be exploited.