Changing ordering of authorship in a proceeding
Here is a quite frequently occurring extreme case which may help you evaluate your own situation:

As @JonCuster also mentioned in their comment, there is always plenty of time between the abstract submission and the presentation itself, and the work typically evolves a lot during that period. Imagine it evolved such that new collaborators were added. Adding new names in there may change the list of authorship a lot. Imagine also that you actually submitted the manuscript by the time you present it, of course with the co-authors on it. In the conference, strictly adhering to the list of authors submitted in the abstract would mean removing your new co-authors, which should be out of question as it is best practice to give them credit as co-authors of your work. Thus, the list of authors can definitely change during the actual presentation as one is usually entitled to also present improvements to one's work. Whether it is common in your field, I don't know, but my guess would be that it is not too rare.