If you wanted to go back to Opus 9 then you'd have to reinstall Opus 9, yes.
That shouldn't be a problem, though. Just backup your Opus 9 config before you install Opus 10, in case you want to go back to it.
(If you have any other questions about Opus 9 <-> 10 installs, please start a new thread so we can keep this one on-topic.)