The length of the term the surety is willing to write changes from surety to surety.
I represent one surety that offers a 15% discount for a 3 year prepaid bond. They also seem to have the best rates, you can apply for them online at the following link:
http://www.jwsuretybonds.com/mortgage_quote.htm
I have never heard of a surety cancelling a bond for an invalid reason. It is very rare for a surety to cancel a bond mid-term period, however almost all require a mid-term cancellation clause in the bond form to consider writing it. (this is now somewhat of an industry standard)
Let me know if you have any further questions.