is there any way to "uncomplete" your entire atlas in standard?

I accidently completed a legacy map on my atlas filling my atlas up in standard. Is there anyway to reset it back to it's completely uncompleted status? I have sort of a self-challenge I want to complete and having my atlas show completed maps i didn't complete in my personal challenge way screws up the whole system.

No, there isn't.

You'd need to create your character in a different league, e.g. SSF.

It's not a free solution - but if ≤ 50 days would be sufficient, you could always create a custom league for this purpose. Their primary goal is to allow modifiers (e.g. no selling to vendors), but those are all disabled by default and you don't need to enable any of them.