First, be aware that ANY carb you add may (or may not) cause a very temporary spike in appetite; imagine your body saying, "Finally!"
With that in mind, you might want your first serving of increased carb to occur in a situation where eating is controlled--only cooking a small serving, or thawing a small amount, etc. If you don't have an unusually strong impulse to eat more, then choose whatever you like after that.
I think either tuber or fruit could work just fine. If appetite could be a problem, pick something you'd enjoy but don't usually go nuts over. For example, I find starch palatable but not delicious so it would be the better option if I'm starved for carbs--fruit I'd go nuts over--so a little converted rice or sweet potato won't cause an appetite spike but a grapefruit will.
After a few moderate servings of whichever food you choose the first few times, your reactions should settle down and you should handle whatever carb looks/sounds good.