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There did use to be a downgrade script...unsure any more...where it is...yet you can also take copies of all information as tables...mysql....
one for each section as in all tables for the forum, one for the news section.... ect...
Then first install 2.0.16 with all these modules, then when they are installed...over write the tables data so the information is now placed into the installed modules....
As for problems with 2.2.4, we just upgraded from 2.0.15 to 2.2.5rc2...and found it more stable not less.