There is no one tool that will do what you are looking for. However a combination of Winrunner or even some Document Compare tools which will compare old and new XML files. However like all testing you need a plan ie what do you want to test, in this instance what functions or business process do you want to test. From this you can then work out what can be tested using specific methods or tools. You may have to take a step by step approach that is do some test manually and at the same time build your regression test bed which can be used in the future.
There are many ways that you can carry out this testing.
One way is - to test the API's of the product.
Call the api with the required parameters and verify the expected result. Change the parameters so that you cover many combinations. This way you can test the functionality of the product. I believe there is no such tool to do this. You need to write code, you can do this in java also.
You can either hard code the parametrers(inputs to api) in the .java file or you can store in some properties file and read the values.
Create an object of the exposed java classes and call the methods with diff parameters.
The other type of testing is - the database verification. Once again you can call the api's and verify the database values to check whether the database is being updated properly or not.
You can even automate this, call the api's and fire a query to the database and verify the result.