Inherited CI 1.5.4: Rebuild or update?

[eluser]rubysue[/eluser]
Hi all,

I have been maintaining a CodeIgniter 1.5.4 application for a little over a year now and am fairly familiar with the framework. It is serving our purposes well, but we need to redesign (new CSS/theme) it with a new rebranding.

I have a limited time to accomplish the redesign, but I think I can get the results we need (including a responsive design) with CSS and Javascript, leaving the MVC components intact. I wonder, though: Should I update the code base, too?

Second question: What workflow do you use for development? I normally use a MAMP/LAMP stack and Git for version control/sync between dev and production. I currently access the CI application using FTP, but 660 permissions prevent me from viewing or downloading several essential files, like user.php. While I could change the CSS on this semi-locked production application, this could be catastrophic without a backup or version control. It would feel much safer to at least get another site (even in the same antiquated version of CI) up for development first.

Any thoughts or advice?

Thanks!