That's under Settings>Servers>Distribution Points (Distb point)> Edit Server> File Sharing
These seem to have been set up during the JSS install and the passwords assigned automatically (?) I guess- is there a way of deriving the original passwords or - if I reset these account passwords and then re-enter them in Settings>Servers>Distribution Points (Distb point)> Edit Server> File Sharing will this have any negative impact?
Those are the user accounts that the JSS uses to connect to your file shares (Distribution Points). You can change them at any point so long as the associated user accounts are updated on your file sharing server.
... and yes, changing the passwords on only one side of the equation (JSS or your FileShare) will break quite a lot of things until you sort it though there "probably" won't be any damage.