We just started using the same URL parameter to capture the original and current values - for approximately 5 different parameters. Basically, we're capturing the original and current values for all the URL parameters that Google URL builder supports and it's working fine.
We're using a custom tracking cookie to store URL parameters in the visitor's browser so that might be helping. If you don't have a tracking cookie to store the URL parameter, then if someone hits your landing page with a URL parameter, then navigates away from the landing page, and then returns to the landing page, the URL parameter is lost. If your form is set to read a URL parameter, MKTO can only read the parameter if it is present in the address on the page. So you need to have a strategy for handling the scenarios when people click a link, hit a landing page, but navigate away from the landing page, before they submit the form.