Launching version 2 of a large WP site (Already live)

Here’s the scenario:

I’m creating a new WordPress Theme for an existing WordPress site.

• It’s a WordPress MU site
• There’s 2 sites total
• Each site has over 50,000 posts
• The database is over 2GB in filesize
• There’s at least 50 posts made per day

Currently I have a development version of the website where I’ve been working on the new theme. The data has quickly become outdated, however it’s served its purpose of allowing me to design the new theme.

My problem is this:
Before I launch the new site they (the client) will need to configure their theme. Tasks such as uploading an image for each category (custom code), setting up slideshows (custom post type), configuring widgets, etc. I feel this must be done on the live site, but in a transparent manner. Trying to merge the development database with the live database would be a mess.

To accomplish this, I’m considering adding the new theme’s functions to the current theme. This would allow for them to work on custom post types and other content which doesn’t affect the old theme. I plan on only allowing admins to view the new theme by modifying a theme picker plugin. The final problem is widgets, since you can’t manage another theme’s widgets. My solution for this is I will register the new themes sidebar in functions.php of the old theme. They’ll then be able to configure widgets for the new theme, without ever disabling the old theme.

Nothing about this is ideal, but if anyone has similar experiences or ideas they would be appreciated. Launch time is in a month or so and I really can’t think of any ideal solution.

Thanks