I'll be really interested to hear how your clients react to this change. At my last job we worked with a number of contractors who were always paid for a minimum number of hours, but these folks had very specific knowledge in their field and would not have been easily replaced. I can see where this would be the case with you, too, if you have a great reputation with your clients!

Also, if most of your clients already meet your minimum # of hours without trouble, then this may not bother them very much. They won't expect that they'll ever need to pay you for un-used hours. It will probably be an issue for those who usually use fewer than 10 hours, but it sounds like those may be the ones you're trying to weed out.

On the other hand, if you want to keep some of the clients who come in under 10 hours every month, then perhaps Kimberly's suggestion would work better, where they pay for a specific number of hours up front and you use all of the hours up before they buy more?

Keep us updated! and Good Luck!