Transitioning from being a bespoke development to a COTS development house

Question

Currently one of the major applications that our organisation produces would be regarded as bespoke software as it is specifically designed for one specific client organisation.

However, we have specifically retained the intellectual capital on the project and a number of different organisations in different countries (with different languages and character sets) now look like they want to purchase our software solution. Our current approach to development passes 9 ½ out of the Joel Test and is improving. However, our approach is all about satisfying the requirements of one specific client and responding quickly to their needs (regardless of whether something is a bug or a change request).

If we want to transition to producing more of an 'Off the Shelf' approach with multiple client organisations---rather than just one client organisation---on a common code base, what are the major differences and what pitfalls do we need to be aware of to successfully make the transition?