Is there a technical reason for the September 1 format change?
No. The format change is being made for legal reasons, not technical reasons. MIT wanted to bring out a version of PGP that would have the support of RSADSI. RSADSI would not lend their support to a product that fully interoperates with PGP 2.3, which, when used in the United States, potentially infringes patents licensed to them by Stanford and MIT. The intent of this format change is to discourage people from continuing to use the earlier software, which will mitigate the patent-caused problems that have hampered use of PGP within the U.S. The time delay between now and September is to give people adequate time to upgrade to the new software.