I misspoke, unanimous consent isn't required, but it makes the process much smoother, and is considered the "normal" means of ratifying committee assignments.

For Diane Feinstein to resign from the Judiciary Committee and for appointing someone else to the committee right now requires unanimous consent, and the Democrats are not going to get that from the Republicans right now. Regardless, the act of replacing someone on a committee is subject to debate and can be filibustered. If Diane Feinstein were to retire or die, that's it, a 10-10 split in the Judiciary Committee until the next Congress, so long as the Republicans refuse to play ball.