Maybe I am being naïve, but I really can't see how this will accomplish very much, unless these agencies are lying and backdating the hire date. Which is of course, possible.
When Trump and his staff assume control, it seems that it would be fairly easy for them to say that all job offers and hires which happened between election day and January 20 are hereby rescinded. This would be especially effective if a personnel audit revealed that overall staff increased by 20%, 30%, or more during that time frame.
I doubt this will stand. I doubt it will have much effect at all.