Don't request a refund. For one thing, refunds are for overpayments, and you underpaid.
If the applicant status is large entity, and you have not specifically requested a change of status to small entity, then the PTO should assume that you underpaid and should automatically deduct the difference under the preauthorization. Best thing is to call and ask what to do, but I think you should be OK without doing anything. Just check and make sure that they deduct the right amount.
Well, I'd prefer not to be in a situation where we request to be charged an additional $75 and have to monitor the status to make sure they charge us the extra fee. Rather than waiting around, I think I'd rather just pay the $150 now and ask for the refund of the original $75. That way, at least I'll know that the fee has been paid and we don't have to keep monitoring the status of when they will charge us the extra $75.
Also, I did call the USPTO twice regarding the first-month vs. second-month fee issue, and unfortunately I got two seperate answers. One agent told me we would need to pay the second month fee, the other agent informed me we would only have to pay the first.