I've made a suggestion before about allowing an ala-carte selection of channels in the next-tier package, but it was shot down to hell. For example, if you are in AT200 and want a channel or two from AT250, you could pay, say, $3 for one AT250 channel, $5 for 2 AT250 channels, and anything more than that you would have to upgrade to the AT250 package. In order to select a channel from a higher package, you would first have to subscribe to the package level just below it.
I think you are on the right track about having to be in the next package down. And also a limit on how many you can choose before needing to go to the next package. I'm not sure $3 is realistic, I really think $5 will be the minimum. It might be you could chose one channel of a list from the next package also. I say $5 because the carrier is not going to be interested in you keeping the lower package. They have to balance getting some amount of money more from a subscriber, as opposed to getting them to want to move to the next package.