Upgrading my phone and plan

I currently have the standard iPhone 15. Unfortunately, I had a small crack develop in the bottom corner of the screen, and it has now started spreading what looks like black/“ink-like” damage throughout the bottom portion of the display. It’s still mostly contained to the corner, but this morning my phone started flashing a bunch of different colors. I can still use it, but it’s definitely becoming a hassle. Because of that, I’m considering upgrading when the new iPhones are released this September. I’m currently with T-Mobile on the Essential Savers plan. My bill used to be around $106/month, but it recently dropped to about $96/month, so the difference isn’t huge. I also recently got a much better-paying job, so I’d be comfortable paying around $150/month for my phone plan if the upgrade benefits make it worthwhile. The important part is that my iPhone 15 will be completely paid off next month, with the final payment scheduled for September 13th. Since the iPhone 18 lineup is expected to launch around that time, I’m wondering how upgrading would work. Would switching to the Beyond Experience plan on the day the new iPhones are released make me eligible for T-Mobile’s best upgrade/trade-in promotions, including potentially getting an iPhone 18 Pro or Pro Max? Or would I need to switch plans before the release to qualify for those promotions? I’m not sure if Essential Savers is the best plan for what I’m trying to do, so I’d really appreciate any advice from people familiar with T-Mobile’s current plans and upgrade promotions. I’m open to changing plans early if that gives me a better deal on the iPhone 18 Pro/Pro Max. Thanks! 🙏