I really don't see why that should be necessary - it must be quite common for site managers to want their admin level users (in my case customers) to have access to only some of the admin functions as they either have no need to access some of them or don't have the level of technical understanding. For example, you may want the admins to be able to manage users, but you wouldn't want them messing with SIte Redirection or Device Preview  (both of which IMHO should be on Host in the first place!).
Is there anyone who has an approach that doesn't involve creating, copying and  deleting pages i.e. a simple move as originally asked?

Which is pretty much what I have ended up doing. :)   I also took care to re-order the TabOrder settings for both Host and Admin pages. Result is what I wanted. I should point out that all my sites are built with a single portal so there is no real distinction for me between the applicability of Host and Admin level settings. YMMV.
I did this direct in  SQL Manager on the server with the site offline, however I believe it wouldn't be too hard to script this via the SQL page if necessary.
Will be interesting to see what happens when I next upgrade the site, however!

Don't try this at home kiddies if you don't have a backup!

Edit:  noticed between your original reply and my response you'd corrected the PortalID reference from -1 to NULL so adjusted correspondingly.

G