Yes, if you create an input box for your variable, you have a list of options where you can set restrictions of what data the variable can contain. It seems to be saved with the variable even if the input box, for example, is deleted.
BI Architect Consultant
I know this post is old, but I was looking for a solution to this problem and didn't find it, so I thought I'd post what I found.
I don't know if this is your problem or not, but if you go to the Variables tab under Doc Properties check to see if the variable you are trying to edit has an Input Constraint. This has kept me from being able to edit the value of the variable. (obviously).
I think it is a bug that you can remove those constraints and it still doesn't allow you to edit the value. But here is a workaround that worked for me: Change the variable to "No Constraints" and save the qvw, close it, and reopen it, I am able to change the value.
Hope that helps.