New tax legislation vs migrated tax to R12 restriction

Problem Description: We have migrated to E-Business tax (Canada).We are using the STCC Determining Factor Set Name, Direct Tax Rate Determination rule, classification code which represents tax groups from 11i. The tax rate for the provincial government will increase on January 1, 2011. However Contracts signed before that date will keep the old rate even if expenses are incurred after December 31, 2010. What is available to us to track this new exceptions that will work with our migrated system so that the calculation of taxe is good.
Can we modify the direct tax rate determination rules and how?
Can we use the tax exceptions for products?