Lion-converted Apple Mail Cannot Be Used With Snow Leopard Boot

Usually when upgrading the operating system (OS), one can assume that key business and personal data will remain intact and usable under the previous version of the OS, simply by rebooting under the older OS. In my view, my data is stuff that should be LEFT ALONE by an OS upgrade, not tweaked and whacked.

With Mac OS X Lion, Apple Mail forcibly converts the mail structure as soon as it is run, rendering the mail unusable if/when rebooting under Snow Leopard. This might have been the case with prior releases also, but the downside remains the same.

Especially for professionals, this forced conversion is a serious problem:

1. Upgrade to macOS Lion.
2. Open mail (since one needs mail every day); Mail converts everything, new messages arrive, work is done.
3. Discover that macOS Lion has a compatibility problem with key software.
4. Big problem— how does one revert to macOS Snow Leopard (even with a clone of the Snow Leopard boot drive), since all recent mail activity is now part of the Lion mail, and can’t necessarily be retrieved or re-sent without substantial effort to save/import, etc.
   

This is a terrible approach; it is possible to write software that works on the existing format and needs conversion only for new features— make it a user choice.

I use mail all day long, and thus a dual-boot system that can share my mail is not an option.

Mac OS X Lion: no choice— to use Mail, it has to be converted and afterwards, no dual-boot option works for mail