You would have to redo the whole spell with triggers. Ones like these are sort of complicated since you have to keep track of a bunch of variables over time (can get messy with GUI if you are not used to it).
If you use JASS, you could use a timer that damages all the units in the area every interval as long as the hero is still channelling the spell. You calculate the damage you need to deal based on each unit's maximum health and then deal that much damage with the damage type you want. You would have the radius in the timer function and you would just have to make it match the spell's radius.
In GUI, you could build it with two triggers. One to check when a unit casts the spell and another one to do the periodic effect. It would work basically the same as the JASS version except you would have to keep track of all the units casting it properly so that you don't break it when lots of units are casting it.
