Sprint and nextel coverage?

Asking again because I didn't get any answers. My parents were nextel customers before sprint bought them out. Now they are under sprint contact but they still have nextel sim cards in their phones. They are looking to upgrade and now sprint is switching to phones without sim cards, so their nextel ones will get tossed out. I looked at the sprint/nextel coverage maps on the sprint website and even tho sprint owns nextel, the coverage is vastly different in our area with excellent coverage via nextel and next to no signal strength via sprint. Am I understanding this correctly that if they upgrade they are going to have horrible coverage?