Diego - your icon says you are "deactivated" so I'm not sure you're still there. I normally agree with Sandy and he's right that you are changing the parameters of the question. But I would address your modified question in a different way (and I think it's a valid question). If you previously had an "Opt-In" field but no "Opt-In Source" field - the scenario you describe could definitely occur. So what I would do is populate the Opt-In Source field with "Unknown" or "Prior to <Date>" or something like that for any people where Opt-In was True and Opt-In Source was empty. Then going forward the value in the Opt-in Source field will be correct. If you want to then backfill it for your older people, that's an exercise you'd have to figure out how best to accomplish given whatever data you had available to you.
You can also add on filters of "data value was changed" and "lead was created" (with "or" logic) plus some date constraints you are comfortable with which will allow you to only focus on new opt ins.