How to manage the versions of databases ?

Hello, In my company, we sell a huge software that uses huges databases, and we modify the scheme of the databases almost every 3 months. This is very complex to manage for our customers, and we have to manually make big changes in their databases. We are looking for a way to manage rightly these updates, and we already have some ideas, but we can't imagine being the first company to deal with that hard problem. So may you give us some URL pointers to web pages explaining how it's usually done, or explain us the theory of updating a database model. I suppose that some tools exists, so perhaps you know one of these tools. Thank you in advance. -- Nicolas Ecarnot | E-mail: nicolas.ecarnot@bigfoot.com