We've recently installed CiviCRM 4.6.6 and we're on WordPress 4.3. I've got a major problem with CiviMail (see HERE. No one has responded in six days. Is there backwards compatibility? Would it be safe to install 4.5.8?? At this point, we're just using the Contact table. I'd rather not import all the data again, since I could only do 600 records at a time (the command line doesn't support groups or dup checking). I've got to get this fixed...someone please help me!! thanks...jim
The recommended method for downgrading is to restore to the backups you made before upgrading.
For contact data modified between when you upgraded and now, you'd need to either export your changes and import them to the 4.5.x system, or keep the upgraded system running and copy the data over.
I would not expect downgrading the DB directly to work, and I would expect manually reversing the DB changes between versions to be more complex than fixing your mail issue.