Changing some XML tags names but leaving unchanged values between them

In one of my XML file I need to find and replace some opening tags names using regex and Notepad++. Also I need to leave unchanged every text between them.

Example:

<uri>http://ift.tt/1D31DCg;

I need to change into:

<image_big>http://ift.tt/1D31DCg;

For some reasons I can't just change uri tag, cause there are others closing tags like /image_small in the document (opened with uri of course).

I tried to change it like:

<uri>.*?</image_big>

But I don't know with what I should replace it. I tried with:

<image_big>\1</image_big>

but result is:

<image_big></image_big>

without any text inside.

I need your help. I'm not good with regex.