I see there are both daily and production builds you can dl to update Houdini. I am using a Mac version.

Does either file entirely replace the older version or can you switch back to it if need be (with the license inract)?

Will the update use the licenses a already stored on my Mac from the old version? I am worried the license will stop working and will require me to go through another complicated licensing procedure.

I have 32 versions of Houdini currently installed (I should get rid of some )
from H16.0 to 18.0
all are unique and run on the same licence.

you should be fine installing a daily build along with a production build.