There is pretty much no reason that these shouldn't update when your number of shards update. If they're worried about balance (ie, you cast them early and then they get upgraded for free as the game goes on) then make them more expensive or tie the upkeep cost to the number of shards.
Otherwise it trivializes a large part of the shard mechanic. Magic is supposed to be tied to shards and owning shards to increase your magic power is supposed to be a viable gameplay strategy. If your power doesn't naturally ebb and flow with your ability to conquer and hold shards, then the mechanic (tying spellpower to the number & type of shards) is essentially pointless.
Look at it in reverse. One of my previous games I had 10 air shards. I cast a few air based buffs, then lose 9 of those shards. Yet I'm still just as powerful, buff wise, as when I had 10 shards, because my buff strength is not tied to the number of shards my channeler has immediate access to.
Edit: My larger point is that treat this as a bug and make enchants update naturally as your number of shards changes.
