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 ...
https://premium.wpmudev.org/forums/topic/upgrading-from-single-site-install-to-multisite-install-on-same-host
https://premium.wpmudev.org/forums/topic/stand-alone-domains-within-one-installation-of-wpmu

This included:
https://premium.wpmudev.org/wpmu-and-buddypress-installation/creating-a-network-by-enabling-wordpress-multisite/
https://premium.wpmudev.org/wpmu-manual/network-options/
http://codex.wordpress.org/Create_A_Network

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 http://happy.yoursite.com then using our Domain Mapping plugin you "Map" or your client can map (if you have given them the privilege of Domain Mapping) the TLDhttp://www.anothersite.com directly to http://happy.yoursite.com"

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