I have 2 ways to do this -
1- At the end of recording - add a "Request Data Manager" and use it to delete the fields you want to be anything. Downside is the values for these fields in the response will be static instead of magic strings.
2- Editing the VSI in workstation, at the bottom of the "Request Data" tab, there is an icon that looks like a notepad and pencil - "Do a mass change of request arguments" - if you pick one field you want to change, and set it the way you want with "Anything" then click this, it will let you change all of the matching fields in the VSI in one click.