Hi Tucson. I don't often make chocolate chip cookies, but make many other types. I assume your "I Can't Believe It's Not Butter" is one appropriate for baking and frying, as opposed to one only suitable for spreading. There is a difference that would truly affect the end result. Even so, if your "I Can't Believe It's Not Butter" is in a spread form vs. a stick, there is surely more water content in it. That would affect the texture of the cookie. My strong guess is that using a spread vs. a stick margarine would result in wafer flat hard cookies rather than chewier ones. Maybe I'd be wrong. You can try it.

I know that the chocolate part of chocolate chip cookies is often the main appeal, but the brown sugar, vanilla extract, and butter aspect is also crucial, giving it that delicious raw dough many like. My advice is to use dark brown sugar instead of light brown sugar, if there is a choice. You would surely want the extra molasses flavor (that is more present in dark brown sugar) to shine more since you would not have the benefit of the natural delicious flavor of real butter that margarine usually lacks. If you like nuts, the flavor of some walnuts, especially, may also help. Plus, they add some more fat (healthy fat) and would help the appeal if the cookie baked flatter. If the margarine also has a higher water content, perhaps a wee bit of extra flour might be needed to get the dough the right stickiness. I'd also be sure to use size large or even extra large eggs, no smaller. Eggs are a form of leavening. Or another strategy may be to add 1/8 tsp or less more of the baking powder/soda. It's possible that the cookies may be a little "cakier" than usual vs. chewy.

If you take any of the above advice, please forgive me if your cookies don't work out.