if you shift your ground in an argument or a discussion, you start to express a different opinion.
He's impossible to argue with because he keeps shifting his ground.
shift (one's) ground
To adopt a different viewpoint or opinion. I was shocked when Liz suddenly shifted her ground and started seeing things as I did.
shift one's ground
Fig. to change one's opinions or arguments, often without being challenged or opposed. At first Jack and I were on opposite sides, but he suddenly shifted ground and started agreeing with me. Jim has very fixed views. You won't find him shifting his ground.