A new coach has zero obligation to honor any walk on commitments, and for that matter any scholarship commitments (past one year per the scholarship papers). Even if they do honor a walk on for a year, they will replace that walk-on with one of their own quickly.
If a school makes a coaching change, they tend to announce this change immediately at the conclusion of the season because they want to 'save salary' by getting the current coach off the books. Athletic Departments save money by not hiring a new coach until later in the winter or spring. In the college coaching ranks, Thanksgiving to the New Year Holiday is the Firing Season (not the Holiday Season). Sometime after the New Year, most athletic departments start in earnest the interview processes to hire a new coach - This hiring process time frame depends on how much the school values volleyball. Unfortunately, it has become common for DI athletic departments to let a volleyball head coaching position stay vacant for months because they can save budget money.
(this has always amazed me about athletic departments; they will let a soccer or volleyball or softball position lay vacant for months on end, to save a few bucks in salary budget; but, they will hire a multi million dollar salary football/basketball coach, and after hiring a consulting service for $50k+, within a week of an opening!)
As a walk-on, your VolleyDaughter is completely at the mercy of the coach; new or current. I get very nervous when families commit to a DI walk-on situation because there is tremendous uncertainty and zero stability. The only time I do support a walk on, is if the school is the perfect fit and the athlete would have chosen that school no matter if they played sports or not.
What I suggest you do is be proactive and not reactive - If you feel the coach may be on the way out, start putting together a list of potential schools. The NCAA DI and DII recruiting process is never ending and there will be plenty of scholarship opportunities after the club volleyball season goes full tilt in January, in addition to walk on spots. Should the coach be fired, then immediately start contacting this list of schools to explore opportunities.
Let's say the coach is fired, and the athletics department hires a new coach in February, and this new coach begins duties in March. Now, it is halfway through the club season and you still have to figure out if this new coach wants your PSA on the team and, more importantly, does your PSA want to be on this new coach's team? Each coach will have their own style, and as your daughter's life will be ruled 24/7 by this new coach (life of a DI volleyball player, walk on or not), she has to make sure this new coach is a good fit.
Don't wait until March to start reaching out; if there is a change come the Firing Season, have your list ready to go, start contacting new programs and give yourself the ability to make the best choice, not the only choice.
Good luck and I hope you don't have to manage this unfortunate, yet more common, scenario!