Correct - If you want to get the lower rate you saw when you added them to your cart, you'll have to pay that by time final payment is due and cannot use OBC to pay it. If you remove it and pay w/OBC onboard, the rate will be at the higher current rate. So, no, you cannot have it both ways.
And if you have them removed then decide you do want to pre-pay and add them back, they'll be at the higher rate.