Generally, I haven't heard of any problems from our students about being noticed by the proctor to exchange an old scratch pad in for a new one. However, ManhattanGMAT strongly recommends not taking any time away from the exam because timing is one of the most important skills the GMAT tests. Therefore, I would practice saving space on our laminated booklet. If you're completing question 33 out of 37 on the quant section with the pad, then perhaps writing smaller or cutting down on the steps you write out on the pad will help you squeeze in those last four problems.
As I previously stated, while I haven't heard of any troubles, exchanging your pad for a new one is an aspect of the exam that is completely out of your control and any time taken away from completing the exam is time wasted. If, however, you can reign in your scratch pad usage, you can remove the entire question of waiting for a new pad, thus bringing this aspect of the exam under your control, making it a non-issue on test day.
I hope this helps!
Student Services Associate