You guys on here are so awesome. I'd wondered if a bit of code could address this, but it would have taken me hours to work out how to do those input/output pieces - thanks so much for the screenshots!
I have my solution working now after a couple of hours doing and testing the workflows.
@PBaxter HubSpot will prevent you from copying fields of different types over to one another.
Alternatively, you could change your initial string data to be in positive or negative value formats - and change the text field to a number instead. you could assume that positive values mean increases and negative values mean decreases. That would then allow you to copy the value to a number field.