If I upgrade, will I get a refund or credit for my current monthly subscription?

This depends on the method of payment you are using.

If you paid by credit card: 

You will pay a reduced amount now getting an instant credit for any remaining time and stay on your current monthly billing cycle.

If you paid by PayPal:

You will pay the full month now and begin a new billing cycle. You will receive a refund from PayPal for the remaining time on your current billing cycle. The refund usually takes 2-3 business days.