After screening an aged oil poly floor, can you recoat with water-born poly with hardner?

I do all the time. Of course the best answer is try a spot and then try and scrape it off. I don't know your products and neither of us know the products on the floor, or the maintenance products used on the floor in the meantime.

You can re coat over oil based poly , applying most any water based urethane.Rule #1 is that the existing oil must be fully cured which can be 3 weeks of perhaps up to 5 or 6 weeks.The basic re coating rules still apply. No wax or oil soap products can be present on the existing coats. Coating over floors with months or years of wax or soap build up will cause problems.

If you are confident that your existing floor is clean and free of wax,polish,etc screen the floor , vacuum thoroughly and if you have one, use a microfiber cloth to tack the floor. The microfiber cloths out there are outstanding.

When you're comfortable with the floor being clean , go right ahead and apply your new coat of poly.