Yes. There are really no differences between child and parent portals except for the "http alias"

If I am understanding you correctly, all you really need to do is add the new TLD URL as an http alias to the ("child") portal in question. (Host | Portals | <select portal, scroll to bottom to manage http alias).

Internal to DNN, that's all there is; as long as IIS on back to the domain name is all set up correctly.

Once done and tested, you can remove the old ("child-type") http alias.

I generally create portals initially as child portals, develop them and what not, and then aim the URL at the DNN install and add the new http alias at go-live time.

hth -