Unfortunately, I'm unaware of any way to call a field's previous value as a token or otherwise in flowsteps. Yeah, seems kind of odd that you can check a field's previous value in data value changed steps, but can't call it directly, but I'm pretty sure that's how it works out.
Would also be nice to be able to call an 'undo data change' on a column. That's two ideas someone should go ahead and write up and post.
Hopefully someone else knows THE TRICK. But if not, this is a big reason to always export a list of any values you're getting ready to change in bulk with a flow action. If something goes haywire, you then have a list of previous values you can correct with a simple re-import.
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