Perhaps one option is to have 4 separate subscription pages so you can target specific lists, one at a time.
Then your Sub Management form will assume the person is returning and let them change all of the options at once.
Thanks for the suggestions. Unfortunately having unique subscription pages for each of our newsletters isn't really an option. Besides the fact that it would force our subscribers to fill out essentially the same form 4 times to get all of our newsletters, and we want to make it as easy on them as possible, we also want to promote all 4 at the same time so that our subscribers are aware of all of our options.
Can you explain your second option a bit more? If we change it to Blank/Yes/No, then wouldn't we have the same issue where we're potentially overwriting a Yes (or a No) with a Blank if the subscriber leaves it blank when subscriber?
Did you manage to find a solution for this?
It seems pretty straight forward to me that you want to have some data fields only update if the value is not blank.
The only way I can think of doing this is with a 'mirror' field in front of the actual field. It's a cumbersome solution for something that should be pretty straight-forward.
The way that would work is that your mirror field would be updated. With a Smart Campaign you can update only values that are not blank.