just set the new template as default, and move your modules around as needed to fit the new layout. The menus and content you've already setup should work pretty much without any need to make modifications.
The problem is that i can't get rt_firenzie.tgz installed in joomla 1.5RC4. I do have the file rt_firenzie_j15.tgz, but in that case i get an error saying missing some files.
Can you please tell me how to install the rt_firenzie_j15.tgz file?
You shoud use the _j15.tgz file. What errors do you get when you install it? Also you could just extract the template locally and ftp the rt_firenzie_j15/ folder into your <joomla>/templates/ folder