Not about being half empty or full. It’s about science.
Think of it this way, you can put cake in a metal tin or a silicon one. The silicon one is flexible and therefore can adapt to fit a more varied shape of cake. Thankfully our vaccines teach us to create antibodies which are more like the silicon. They provide multiple spike protein reference points which mean our antibodies recognise them and target them. It means there is some flex to changes in the spike protein. Just like the silicon container is more flexible to different shapes than the metal, our vaccines allow us to be flexible to different spike proteins.
But within reason. You simply can’t fit something which is round into the silicon tin if it’s rectangle. Even with the flex. It’s the same here. If the spike protein changes so significantly (and in this case it has, which is why there is concern) the vaccines can suddenly very quickly become irrelevant.
It’s not a simple yes or no, though. There is varying degrees of recognition. It might still recognise a few parts of the protein and so some peoples bodies may recognise it and deal with it. Some won’t. This is how you end up with a reduction in overall vaccine effectiveness across a population.
So the key question is by how much the change in the spike protein will reduce effectiveness.