Can we take a definite stand in the GD and then later on during the GD, switch over to another stand?

Yes, as long as you do it correctly. In a GD, it’s quite probable that someone else’s counter-argument will persuade you of your point of view. If this happens, it’s better if you accept his point and explain to the group how your prior argument was correct in a limited context but not in a larger context, and how the new argument is correct in a broader context. Naturally, it is best not to make any hasty remarks for or against a topic until you have thoroughly researched the facts. Taking a stand on the spur of the moment will almost certainly get you into trouble. This does not imply that you should remain on the sidelines. You may actively engage by presenting all sides of the argument in a reasonable and logical manner.

From my personal experience from the Group Discussion that I have faced throughout, it doesn’t look good if you look confused while taking a stand or placing your points on the topic of discussion. The idea is to present yourself as a person who can win people to his/her side by making sound arguments in the favour of the side.
But nowhere does it say that you can’t appreciate the points of the other side and agree to some corrections on your behalf, by exactly explaining the reason behind it and not just because you felt dominated by the personality or tone of the person with counterargument. It reflects that you have an analytical and logical mindset with enough reasoning capability to separate points and look at them setting aside your ego, which are signs of a person who believes in getting the job done rather than being stubborn and halting the process