I found a potential solution for this for anyone still needed it. If you already have a new subscribe work flow setup, but your old customers have not yet been put into it this is what you can do. It will take some time since you have to do it one at a time, but it works.
Go to the customer profile for each customer and click on the 3 vertical dots on the right side by their information. Then click “edit marketing setting” when the new box pops up, uncheck “Customer agreed to receive marketing emails” and then click save. Repeat the process by opening up the “edit marketing setting” box again and this time check the box that says “Customer agreed to receive marketing emails” and save.
This should put the customer immediately into your new subscriber automation work flow. Like I said if you have a lot of customers you need to add this will take time, because you have to do it for each one individually.
Hope this helps!