Why is it that once you apply tax updates against your upgraded Copy of Production database, it cann

I am at the end of an Initial Pass.  The Demo (Upgrade Image 9) is at Tax Update 14E and PI15 is at Tax Update 15E and the COP was on 15E.  I want to apply Tax Updates to bring COP up to 15E again based on PI15 but the Appendix: Upgrading with Tax Updates in my Upgrade Job has an important note at the end stating:

Important! When the upgrade is complete, for purposes of running parallel tests to compare results, your production database and the Copy of Production database must be at the same tax update level. Apply any additional tax updates to your Copy of Production database so that they are at the same tax update level. Once you apply tax updates against your upgraded Copy of Production database, it cannot be used as a Source database for future MTP passes. If you have future MTP passes planned, take a backup of the Copy of Production database before applying the tax updates.