I would like Adobe Acrobat 9 to open my PDFs as a default program. When I followed the Adobe prompt to update (pop up), it changed the default to Adobe Reader 10.
Most likely the update to Reader 10 uninstalled Reader 9. You cannot normally have multiple Reader versions installed on the same system.
He's talking about Acrobat 9, not Reader.
Those two versions can co-exist, but it's not a recommended situation. Only Acrobat X and Reader X can officially co-exist without issues.
With any other combination, you might run into problems, like the one described above.
In Adobe Reader x, go to edit-preferences->General. Click "Select default PDF handler" and select Acrobat 9 from there.
That worked fine, thank you.