It will do no harm if done properly. We have a similar setup in central Illinois. Balance your water, close the cover. We put water tubes on the "open end" to keep as much junk out was we can. My pump is below my water level so we plug the skimmer, blow out the line towards the pump (with the valve open and all pipes off) until all the water is out. Then add a gallon of rv antifreeze, air, gallon, air repeat until I get antifreeze out of the valve, close the valve. Put a widget in the skimmer, open up skimmer drain and done.
For returns I hook vac up at pump and start blowing air. Closest return will bubble, cap off, next one bubbles, cap off, last one bubbles, cap off with a one way valve fitting. And then Repeat anti freeze trick from skimmer until I see antifreeze come out of one way valve. Cover all equipment and have a beer.
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