Its not exactly as easy as that Mthiessen, the changes between 2.0.18 and 2.0.19 were too extensive and many mods themselves needed upgrades.
I hope you did the following...
A) Kept a copy of the phpbb forum version you installed.
B) Kept a copy of all the mods you installed including a copy of the "how to add this mod" page if it wasn't included in the download.
C) Kept a list of the order in which you installed the mods.
D) Kept a list of the changes you made yourself along the way.
E) kept backups of the database both via MySql and via the acp of your forums.
If you did good for you! Many people break the cardinal rule of not keeping backups before making changes.
For your situation I recommend one of two options.
Option #1 - make backups of both your database (via your host or phpMyAdmin) and post data (via the acp). uninstall the mods one by one in the reverse order you installed them in including any changes you made on your own. Upgrade the forum to 2.0.22. Install the NEWEST version of the mods you originaly installed and if done right you will be set.
Option #2 - wait for Olympus to get out of Beta, its a whole new type of forum and the mods you have now will not work without being changed again. You'll have to un-install again, use the upgrade to Olympus (phpbb3) package phpbb group will undoubtedly provide and then you'll need to decide what new mods you want from the suddenly smaller list available.
I dont know which mods you have but your first step should be to see if the mods themselves have updates for 2.0.18 to 2.0.19 or later. Sorry for the news but 2.0.18 to 2.0.19 was a biggy.