Its the same as its always been. It's what they did to get rid of solo-lane except in reverse:

Remove base damage and add AP ratios to compensate.

EDIT: To clarify, not remove ALL base damage. Remove some base damage, add a little AP ratio to compensate

This crushes these champs in the support role while not harming them mid. Even though they buy AP as supports, they can't afford to buy nearly as much and can't do it nearly as fast as a midlaner. I'm aware this would hurt their Mid early game a bit, but lets face it. Whats the other option? Let them rot as bot lane supports?

Another problem that needs to be addressed to bring them back to Mid is the synergy between . These two items add way to much damage to these supports when they use any AOE. Supports can buy just these two items and dish out tons of damage. My suggestion would be that Rylai's slow doesnt count towards the extra burn on Liandry's. Only CC that is within a champion's kit should count towards the extra burn. And I mean any champion on their team, not just themselves.

For example,

• Eating an E>Q combo from or a root/knockup from would trigger the extra burn affect. ult would trigger the extra burn.

• Getting tagged by just 's E and getting slowed from just the Rylais would NOT apply the extra burn. Getting autoed once by a plant would NOT apply the extra burn. just pressing E would NOT apply the extra burn

Seems pretty simple to me. The inverse has worked in the past so I don't see why it wouldn't work now