Something has to happen for a trigger to work... I agree, the Data Value Change trigger could work. You can add in a constraint of what the old value was and what the new value is. Here's a quick guide that is pretty handy, although it looks like it might be out of date (there seem to be more now than are in this list): https://community.marketo.com/MarketoResource?id=kA650000000GugSCAS
Sorry, probably should not have entered "trigger" in my post. We are looking for a campaign to run from the field itself. But knowing that some fields can be overwritten, we would like to know if we can pull information fromt he activity log for prior data entries from a field.
I would suggest using logic such as "IF [Field Name] equals [Your Desired Value], then add to campaign". If you are using a batch campaign to add users, it won't matter if the data value changes in the future.
You can also look for a data value change with a date constraint and use the old value to catch those that get overwritten.
If you are wanting to make this a recurring campaign add, you can set it to look for this value, but you'll just have to be sure to exclude existing campaign members.
@Karen B if you have not previously made the choice to append/prepend information to the field -- instead allowing it to be completely overwritten -- then as far as I know there's no way to peek at previous values. You could try using the API to retrace a lead's steps.
You could have a smart campaign that listens for that field becoming a certain value and then have a flow step to add the lead to a static list. Then you could reference this list for a complete view of anyone who has had that field value now or in the past.