What is the effect of switching between conflicting social policies?

• You get some turns of anarchy, which means your science, commerce and culture stagnates and you get a big penalty for production,
• You lose the policies in the branch you abandoned (no refunds).

Actually I think in some cases it still might be not such a bad idea to do that. Especially if you didn't invest too much in that policy and you are not going for cultural victory anyway. Some early game policies matter much less in the late game. For example you can use a bit of Liberty for early expansion and then abandon it for Autocracy.

The effects of switching policies, in pictures.
In this case I already switched from Piety to Rationalism. I lost all Piety benefits, but the Piety branch and all already unlocked policies are still available. It's possible to switch back to these deactivated policies for the cost of 1 or more turns of anarchy and losing the benefits of the conflicting Rationalism branch. This is possible at any time, you do not need to accumulate Culture to do so.

The tooltip reads "Inactive because there are other branches active which block its effects. Click here to switch."

Switching will cost, in this case, 1 turn of anarchy, during which no Production, Science or Gold is produced. I'll also lose the benefits of Rationalism.

Anarchy!!!

After the switch is complete, all my previously unlocked Piety policies have been restored. Rationalism has become inactive instead.

What I've read from other players is that the conflicting policy tree will be deactivated, but the policies you've unlocked in it will be preserved. You can switch back to it at any time at the cost of anarchy. So the original policies are not lost forever.