I read the question in two different ways. The second one was what Aleks has covered.
The first was "I created a page with template X, I should have used template Y, can I convert?". To which the answer is "no" - a template in Confluence is literally a template. When you create something from it, you get a page with preset stuff on it for you to edit. The template has nothing to do with the page after that initial population. There's no link between them, and changing a template has no effect on existing pages.
If you would like to convert an existing page to a template, you can copy the content from the page and paste it into the template editor. There is no automated way of doing this.
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