I realized I should have included a bit more information, sorry. :)
So - SubmitToSharePointList uses a Submit data connection. That means that when the data connection is executed (as it is during the SubmitToSharePointList command), InfoPath checks the validation on the main data source. If there are validation errors, the submit data connection can't execute, and the command returns an error.
The ClearErrors command will clear the validation errors for the session (so you want to use that with care, as you may not want a user to be able to submit a form with validation errors) - you know about the GetErrors command, I think we discussed it in a different post.
ChangeSubmitUrl won't have the outcome you desire for your SubmitToSharePointList command, since the command is already using the URL from the mapping file, not from the data connection. So a rule to just change that URL in the mapping file should give you the result you want.