There's already food/water items in the game that restore a certain percentage of your total health and mana, rather than a set amount. That's probably what we'll end up seeing.

Two simple solutions to mage water/food:

1. Make conjured water/food downrank itself automatically when traded to a low level character, or
2. Make conjured water/food only 1 rank and let it scale with mana/health pool (much like the holiday items do). As in "Drinking this will give 80% of total mana over 30 sec" instead of "Drinking this will give 35000 mana over 30sec".

I wouldnt worry too much about stuff like this

Ghostcrawler replied:

"There won't be a way or need to cast the lower level spells. If you're just worried about losing the flavor of croissants vs. strudel, we're keeping that. At some point you'll just start making the better breads."

Source: http://forums.worldofwarcraft.com/thread.html?topicId=23425636414&sid=1&pageNo=34#675

...I'm not sure he answered the way I would like it. But it does appear that our ranked food spells will still be around. The other spells will not be ranked and scale to level. I presume this also true for our Mana Gems.

my guess (with absolutely nothing to back it up) is that the mage food/water will work like halloween candy, with different ranks giving a maximum amount of hp/mana.

Tooltip e.g.:

Mage Food (level 80). Restores 3% mana and health per second over 30 seconds, up to 22000 mana and 19500 health.

Mage Food (level 2). Restores 3% mana and health per second over 30 seconds, up to 700 mana and 400 health.

or something like that. Doesn't arcane intellect already do this automatically when you cast it on lowbies?

It does, but it's also not a conjured item, but a spell. I'm sure they'll figure something out, it just stuck out to me for some reason.