Hey Edwin, et al, how do I go about converting a stand alone to a slave site? I've seen a few posts on converting slave to standalone, but is the reverse possible??

Yes this is absolutely possible.

The constraints are that you need to have exactly the same extension with the same version between the master and the standalone.
So each extension that are installed in the standalone must exists in the master with the same version.

Of course, you also need to have the same version of Joomla between the master and the standalone.

When you are sure that the PHP Code and the DB structure are identical between the master and the standalone then you can declare a new slave site in JMS corresponding to the standalone website.
When done, you can re-use the standalone "configuration.php" to replace the one that you will have for your slave site. This will give you the DB Content and re-use the standalone DB.

Once you have the slave defined, perhaps that you also have to move some specific files like images, video or other specific stuff. In the case of the Joomla template, if you have perform some customs, it is recommended to create the slave site with the partial sharing of the /templates. So that you could have your specific /templates in the slave that would be a copy of the one coming from the standalone.

That's all.

Thank you, Edwin.