Like wars, one of the things that modern pandemics do is greatly accelerate R and D in the necessary areas. Because its known what has changed in the new variant and how the vaccines work, its much easier to work out what to change and how to change it. Compare this with small pox vaccine which developed from an observation that people who had had cowpox didn't get smallpox. They didn't know why or how.
The covid vaccine is not quite the same as the flu vaccine. To create the flu vaccine, scientists have to predict what the most prevalent strains will be in the coming season and make up a cocktail accordingly. Usually the prediction is a good one, sometimes they miss by a mile. For the current covid vaccine there is no need of prediction. Even if the vaccine becomes less effective, there will be plenty of samples available to work on modifications and people will know what to do.
Human testing, apart from seeing if the existing vaccine volunteers, catch new variant, won't have happened yet. but I bet (and JVT) that in vitro and animal testing is happening now.
I am not saying "hurrah for science it will all be fine now"