The hash function seldom changes, and will be clearly documented in the Release Summary. To rephrase Todd's answer, an in-place upgrade to a new release will not change the hash function. In general a new hash function can be implemented only by completely reinitializing the filesystem.
Think of it like reformatting your hard drive and doing a "clean install" of a new OS on your PC. You then have to do a complete restore of all the data from backup and reinstall all applications (SPs, UDFs). So in many cases, people will opt to wait until they have a brand new system and are copying everything over anyway.
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