I have a 1/2" compression fitted toilet shutoff valve. If I remove this, and the compression ring, is it safe to us a push-to-connect fitting to replace the compression fitting? Has the copper pipe been deformed by the compression fitting so that the push-to-connect will leak?
If possible I would cut the copper right below the compression ring as most times the pipe deforms slightly or worse if it was put on at an angle and then tightened. This might cause it to not seal properly when the push fitting is installed.
Otherwise its a gamble -- but not a bad one, if it leaks just pop off the push on connector and the cut the pipe.
If you have never used the push on connectors make sure you have the little plastic tool so you can pop it back off otherwise you'll be stuck.
Yes, it is safe to do this
However... There is always the possibility that the brass pipe is slightly compressed to the point that the fitting will not seal properly. In replacing valves under my sink I had this issue trying to cap the pipe with a push fitting for a temporary block. In turning the water back on, it was blasting out from one side. I had to cut the water back off, then cut the brass pipe far enough back where the pipe was not deformed. The cap then worked as intended.
If you have enough pipe, it would be better to go ahead and just cut off any possible compromised pipe, and then use the fitting as intended.
Make sure you have the removal tool in the event you have to remove the push fitting if a leak is present.