If it's willing and able to stay attached upside down, for now (and the next week or two) I'd leave it alone.
If it lets go, I'd collect it and put it in a smaller tank/container or even just a big beer mug. Then at least 2 or 3 times a day do a 100% water change with water from the tank. If the nem still looks as OK as it does now the old water could go back in the tank. Or just pitch it and add some new to the tank, just don't use new water in the container with the nem. You want it to gett even better acclimated to the tank water.
That's just my 2 cents worth. Oh, BTW, I've kept anemones in a peanut butter jar for 8 weeks before I put it in the tank. They were rescues washed up on the beach while we were having serious red tide issues. The nems did fine and have been in my 40g cube for about 7 months now.