Except there's also the redirecting spell energy rule. If at any point the wizard wants to stop attacking he can simply convert the spell into a block or a maneuver. No shifts lost or anything.
And just as converting a block into an attack ends the effect, you shouldn't keep the longevity of an attack if you change it into a block or maneuver. He shouldn't be able to switch it back and forth at will.
One suggestion might be to make him roll Discipline to hold onto the energy every round whether he's using the energy or not to keep controlling it, and enforce Backlash or Fallout if he doesn't make the roll. That way, he's still got the power there for free, but there's always the risk--especially if he's keeping up a full-strength spell--that he's going to lose his concentration and thus lose the power.
Right now you're thinking about a fight with mooks, etc. Something that isn't likely to last more than a couple of rounds. What about your big bad? The conflict that you want to last a while and seem challenging. How are you going to do that with the wizard throwing huge attacks, and the mortals dealing less than half that?
The Big Bad ought to be big and bad enough that he can figure out a way to counter it--maybe he hits the caster for a consequence, and tags it to say, "And he's so rattled from the PUNCH IN THE SNOZZ that he can't keep a hold of that fireball anymore." Or the GM can throw a compel the caster's way to discourage it in some manner.
So how's this for a proposal:
A caster can create a multiple-exchange attack by devoting one shift of effect to each additional exchange. For each exchange that the attack is active, the caster must roll Discipline to maintain the hold on the spell, involving fallout or backlash as normal if he fails to make the difficulty equal to the spell's Weapon rating,
regardless of whether he is attacking with the spell that exchange. If the caster is attacking using the spell energy, the Discipline roll will serve as the targeting roll as well.
The caster may not cast any new spells while the first is active. If the caster attempts a separate action while the spell is active, either the Discipline roll or the main action roll must be made at a penalty of -1, similarly to the supplemental action rules. The caster may convert the spell energy into a Block or Maneuver, but any change to the spell's parameters--including converting it into a zone attack or a spray attack--forfeits the spell's remaining longevity.
Example: It is just not Harry's day (but when is it ever?), and some Red Court vampires have come to say hi. Harry decides that he can't make an effective spray attack with his Discipline skill given how many vampires are after him, and he doesn't want to risk frying himself with a zone attack, so he decides to try a multiple-exchange attack. He summons up a Weapon:4 fireball, and gives it three exchanges of longevity, and rolls very well, getting a solid 7 to control, putting a big flaming hole through the first vampire. After dodging a couple strikes, he throws it at a second vampire, rolling a 4 to successfully control the energy, and singes it. Harry decides it might be a good idea to get out of dodge, and decides to devote his next turn to doing so--he decides it's more important to get out of the zone, which has a border that will stop at least some of the vampires, so he decides to make his Athletics roll at normal, and roll his Discipline from the penalty. As a result, he makes it over the border, but only rolls a 2 on Discipline, and takes two shifts of backlash to maintain his hold on the spell. The barrier stops the vampires, though, so on his next turn, Harry decides to let'em have it: He converts the spell into a Weapon:2 zone attack, and manages an Epic roll, cooking nearly all the vampires he'd left behind.
