Your granite guy would need to come back and recut. It will be (possibly) a dirty business (lots of dust).

In general you don't want to cut granite in place if you can avoid it. It's really messy and the person doing the cutting will almost certainly say that they take no responsibility for cracks or damage because you never know. If you already know you want a larger cooktop but can't afford it right away, for instance, then I would talk to your contractor for options. Perhaps some sort of fill material that can then be easily removed. Maybe even have your granite person make a filler piece from the granite he's cutting out anyways and then put in place temporarily. The gap or seam can be filled with clear chalk and when you are ready for the new cooktop, just remove the filler. This is all provided you already know what you want eventually. If not, then that's probably something you should figure out sooner rather than later. BTW, most cooktops have a reasonably standard cutout dimensions for either 30" or 36" so you might not need to decide on the specific cooktop now. You probably *do* want to pick out a couple of likely suspects and make sure you have a cutout that can accomodate all of them. There's usually at least 1-2 inches of "lip" around the perimeter so as long as the edge of the cutout sits within that perimeter the you're fine. Look for the minimum and maximum cutout dimensions for each of your choices and make a cut out that's about 1/8-1/4" larger than the largest in each dimension and then check that to make sure each dimension is less than the smallest "Max Cutout" in each dimension.

Clear as mud? :)

Good luck.

Alternatively, since you are remodeling your kitchen, you can choose a rangetop instead of a cooktop. A rangetop is like just the top part of the range, without the oven, and the knobs are on the front instead of on the top. A rangetop is a "drop-in" appliance, so you won't have to have a cutout. If you want to replace it in the future, just drop in another one. They come in the same sizes that are standard for a range, 30", 36", 48". Another advantage of a rangetop is that it gives you more room for your pots because you don't have the knobs taking up cooktop surface real estate. Also, unlike a cooktop, you also won't have to have your hand near the flame to turn the flame up or down. One disadvantage is that rangetops tend to be more money than cooktops, but you may save money by not having to do a cutout, so it depends.

I'll link to an Electrolux rangetop below, just so you can get an idea of what one looks like (I'm not endorsing Electrolux one way or the other, though it is nice; it's just a convenient link, there are many other brands also).

Here is a link that might be useful: Electrolux Icon 36-inch Rangetop

Many thanks for the comments!

I know about rangetops but don't want to spend that much, and we don't need the additional BTUs or the resulting increased venting requirements. I did not know that the cutout sizes are standardized for rangetops---that is great. In my experience they are not standardized for cooktops. Oh well.

When we got a quartz counter installed where our sink was, the cutout was done offsite and I don't think it could have been changed. Choose your final cooktop to match the one you are installing now and go ahead and buy it. Then store it until you need it. The price will be cheaper than the going rate at that time I bet---but where to store the thing.