If I set a maximum total usage for a coupon, and the usage is met, but then want to increase the total usage of the coupon, what happens? Will I need to create a new code, or will the old one work when I modify it?
When the maximum usage is met, the coupon will become inactive. When you edit the coupon and increase its maximum total usage, the coupon will automatically re-activate (provided that there are no dates or other restrictions in place). Thus, you do not need to create a new coupon code.