I've tried doing it, but to no avail.

I have an existing phpbb 2.0.11 forum with a few mods.

I also have a newly installed phpbb Xtreme 1.7 forum on a new database.

I really don't want to lose my MEMBERS NICKS/PWs & their POSTS/PMs.

Any insight would be greatly appreciated.

I have phpMyAdmin but I am PHP & SQL ignorant.

However, I am "good" at following "good" directions.

In order to do a backwards "downgrade" (even if there are more features, going back is still a downgrade) requires you to compare the database structure. Find the fields that 2.0.11 has and compare them against the fields for 1.7. With any luck the differances are simply "extra" fields like timezone settings and converting will be easy. However, if passwords or other critical features are stored differently you will be in for a hard time. Once you have identified the data changes drop the "extra" fields from 2.0 and run the script. You shouldn't have much trouble dropping fields (make a backup before you touch the data!!!), just view the table and click the "X" or trash icon near the field you need to kill. However, again, if data types have changed you have a much bigger problem and will have to sort out how to convert things (the easier is usually to just reset a particular field/option for all users).