Existing customers should not look at price difference between new and existing customers, you should look at the value you get out of it, and is it worth it to you.
This isn't just a question of whether the higher price might still be worth it. It's about what the policy says about the company. When a provider offers better deals to new customers than it does to existing loyal customers, it is a good indication of the corporate culture and the attitude the company has towards it's existing customers. It is a culture that is geared towards getting as many new customers as possible, while considering existing customers as locked in and so not worth bothering to maintain. Basically they're saying "now that we've got you as a customer, stop bothering us, because we can't be bothered by you." Comcast operates the same way, so I'm not saying Sonic is any worse than they are. But that is an incredibly LOW
bar and I'd have hoped that Sonic would do better.