Hi JamesLaker,
This is a common problem when you create an InfoPath form *from* an existing Web Service or Database. In a nutshell, you can't change the data source. Our advice is to create your XML schema independently and connect to Web Services to submit or query. If you do that. The Web Service data source will be secondary. Because of the limitations around the Database and Web Service derived schemas, we created the Database Accelerator Web Service Suite (DBXL). You take your existing form (with it's own XML based schema) and just submit or query from your database. This way, you can change your main data source schema whenever you want. And, the Web Service or Database schema can also change any time. No problem. The main data source is decoupled from the secondary data sources. That provides more flexible development. You reduce your costs.
We now leverage DBXL for 90% of our InfoPath consulting projects because it makes it tremendously cheaper to develop solutions and maintain them longterm. Customers tend to like that. Given the cheap cost (free for dev version), you really can't go wrong.
If you don't have a database, just create your form from scratch and add the Web Service data connections so that they appear as secondary data sources. Then, you can remove and add new ones without a problem.
Hope this helps!