Problem with switching from single Wordpress installation to WPMU installation

My developer just sent me this ... he was upgrading my Wordpress to a WPMU installation. I sent him the information I received from this forum ...

This included:

Here is my problem ... I DON'T want to lose my number 1 Google ranking. The site is backed up BUT will a fresh installation compromise this??

This is what he said:

While installing the WPMU, there was a previous installation of WPMU on the same Domain and wp. That was installed with sub-directory facility. And it is told in the doc file that it is not possible to change the installation from sub-directory to sub-domain facility once it is installed anyone of those.

Hence when I wanted to install the sub-domain facility with WPMU, it denies to do that.

To follow the guideline I have to get the sub-domain facility with WPMU:

"This is how it works ....
User creates a **sub domain** site then using our Domain Mapping plugin you "Map" or your client can map (if you have given them the privilege of Domain Mapping) the TLD directly to"

Should I go for a new WP installation on it? and then try again? I already have taken backup of your site. ..... Please advise.