Multi Domain + Multisite, existing

Howdy,

For a client of ours we have a multisite installation running, which is working perfectly. Small explanation:

Company has a 'main' company: a domain; maincompany.com. Some smaller company's related to the main one have subdirectory's; maincompany.com/smallcompany

Working perfectly. Yet, there is another company, within that installation; main company.com/alittlebiggercompany and this has a domain mapped to it, http://www.sidecompany.nl/

This side company has several smaller company's again but they will not run under the domain sidecompany.nl. I can use the multi domain plugin, which is working perfectly, but only for new sites. I can not change the existing sites to the domain side company.nl but creating a new one just for side company.nl is working perfectly. I don't want to recreate the existing sites, so I wonder if there's an easy way to get this working for the existing ones. I hope I made myself clear, if not; please ask questions.

Regards,

Danny