Web Application With Multiple Version Support?

We are developing a web solution for our clients to handle their business processes. With reality in check, clients' requirements change over time; sometimes adding a new field, changing the calculation logic etc. When it was still a small scale solution, whenever an upgrade occurs, we will upgrade any existing data to the latest change i.e by having a empty property for it. I would admit this isn't the best of scenarios as old products that previously don't possess certain properties are now 'new' again which doesn't reflect the product correctly.

I am looking for a solution to introduce versioning to this solution before things get out of hand. What I've come up with is like this:

Realising that data is semi-structured (it changes from time to time), we've decided to go with an XML / XSLT approach. Storage of the data will be a mix of columns storing individual properties (e.g. ProductID, ProductName, VersionNumber) and XML columns that holds XML data to cater to fields that will change.

These XML and XSLT will have version numbers to tell the system which version of XML works with the other.Logic wise, we were thinking of developing our business logic to support multiple versions so that old products will still be calculated using old algorithms while new ones will use the new algo. For old products, besides updating them using the old algorithm, we will allow an 'upgrade' option so that they can be upgraded to the new version if they want to. Database wise will remain unchanged since we will store fields that are to be changed in XML form so extending fields should be catered for.

Would like you guys out there to comment on such a setup. I'm concern about performance since some fields are stored as XML, retrieving it will require extra steps, hence the reason why we decided some consistent fields should be on their own for searching purposes etc. Also, would doing multiple versioning like this get really big on the logic side as the program grows?