Someone has to give you the bad news:
You can't. You have to copy/paste and re-edit again in the other mode. When you create a new pages document, you have to wisely chose the one that suits you best. You can’t go back and forth between them nor mix the types in the same document.
Sad but true. ;)
When you have a Pages document, you can always go to File -> Save as Template
After you’ve saved it, whenever you chose New -> From Template Chooser, you have a small section that says “My Templates”:
Your “Custom” templates should appear there.
For the second part of your question, what you need is what Pages call a section. The steps for creating a new section are pretty straightforward and you can find a lot of information on google, see samples here and here (find the part where a new section is created).
In short: Insert a “Section Break”, edit it, change the styles, add what you want (see the links to see tips about different things you can do, like having a different ‘first page’ than the rest, etc.).
Once you’re happy with your “section”, make sure the ‘caret’ (cursor) is blinking in any part of the section and go to Format -> Advanced -> Capture Pages:
The result is a sheet dialog like this:
Give it a name, make sure it includes the pages you want (1, 2, 3 or all, depending upon the complexity of your idea, in your case it would be ‘First Page Only’) and finally save the document as a “Template”.
The page(s) you’ve used to create a Section can be safety deleted before you save the template (unless you want your template to always start with those pages by default), because you have captured them as a section, they will always be available under the new template.
Best Answer
If you delete everything on the second page, the second page should disappear. Maybe the template needs to be removed or there are spaces on page 2 that you don't see.
To avoid a lot of guesswork, the function "View -> Show invisibles" should make obvious what is on the page:
Invisible characters will then show on the page: