From the article I read[1], the nuance was along the line of this -- a person who was previously exposed to the original COVID vaccine or the virus itself (and had an immune response) getting the updated version of the vaccine ends up having the immune response for the _original_ antigen[2], rather than the new one.
Since the immune response is for the older versions of the virus, getting the updated vaccine still ends up having the same effectiveness of the original vaccine.
[1]: https://arstechnica.com/science/2022/02/monkey-study-casts-d...