Updated: Aug 26, 2022
Imagine the gov makes some sort of blunder in their case (ie. an avoidable issue with their plan) and the opp catches it. Then the gov tries to compensate for their mistake by making changes to their plan in the member speech. Shifty, right? That's a case shift.
A case shift is when the gov or opp changes their case after already introducing it. This is illegal. Once you've brought up an argument, you're stuck with it. Even honest mistakes can't be changed in a formal debate, so try and learn from them!
What can you (or your opponents) do to avoid a case shift? A lot of teams try to "clarify" their plan, definition, etc. While this may not literally be a case shift, it still looks shifty, so proceed with caution.
In all honesty, the best way to pull through is to stick it out. Squeaking by a case shift makes it appear as though you are afraid to debate your opponents on the terms you set out. Try to refocus the debate on issues better for your team. All cases have holes; good debaters just know how to make the judge look the other way!