Another question I have explained on StackOverflow - (original link on StackOverflow):
Is it possible to change the Template of a page to Branch Template? When I try in Sitecore 7.0 UI it gives a error message stating to "Select a Template".
Is there any other way to change an Item's Template to be a Branch Template?
Answer: You cannot just substitute a template of an item with a branch template. You might be probably confused by mutual term template
they both use and the fact the both exist undet /sitecore/templates
node, however they are absolutely different in their internals.
Branch templates are just template sub-nodes that are being copied in
instantiation. You may still use tokens and they would apply for every
single item in a branch. But each of the items from a branch would still
origin from certain individual template.
Thus, you cannot just do Configure --> Change
on an item and then select a branch template instead of ordinary
template, that's why Branches node is highlighted with grey color and
even if you try to select any - sitecore will tell you "please select a
template".
What you can do is just to re-create your item from branch template
and (assuming) previous item now becomes a part of your branch - you may
simply manually copy all the presentation details. However an ID of
that item would differ with an ID of initial item (unless you manually
replace original item into a branch instantiated)
I previously wrote an article on how to copy presentation detail manually between items, it will help you with your case:
https://blog.martinmiles.net/post/copying-presentation-details-manually