One of the "unsaid" parts of taking over a program:
What "Traditions" does the program have that the community will be upset if they are not continued?
For example - some schools do a "dance show" after UIL. Some schools always play a certain song at the start of the fourth quarter. Some schools have a different take on a band banquet. Some schools have an "adopt-a-freshman" program. Some have a particular cadence or three that are part of the everyday rotation.
Figure out what SHOULD NOT be changed - and remember that some traditions exist because a director did it for two years, and now "we have to" even though it may not be a real thing.
First, there are three ways to be approaching this. 1. You may be replacing someone that was highly disliked and not respected by most/all. Or you may be replacing someone that was highly popular in the community. Or you may be replacing someone who was polarizing (liked by some but not by others). Or you are replacing someone who was (to be less than polite) forgettable. It is always wise to see what is going out the door while you are going in. Learn what you can to not fall in certain traps. 2. Once you get started building relationships is your first goal... can't overemphasize this... build them with your colleagues, students, staff, booster organization... get to know names and faces. 3. Don't try to change all things at once. Especially if you are replacing someone who was liked or highly thought of. Earning trust and building community in your organization is more important than immediate enforcement of your policy ideas. Think of it this way... you must go slow before you can go fast. 4. Don't put too much pressure on your self. You will feel it in the head position and you have to find ways to share the load. You must be organized and you must be diligent, and you will probably spend more time at the band hall than you did as an assistant, but realize progress and growth in an organization take time. The seeds of the band that you will get next year were planted years ago and it will take time for that organization to really have your true thumbprint on it.
Try to learn everything about the "other" stuff involved with being a head director. Now you'll be dealing with students, assistants, admin, parents, other teachers, paperwork, scheduling, and anything else that will get thrown at you. Being head means you need to keep all your ducks in a row and be at least organized enough to handle all the stuff your assistants don't have to deal with (which usually is not no much difficult as just time-consuming and tedious).
Talk to anyone and everyone you can. It seems like you already have a good start with your mentor. Ask other directors how they work. If you're able, observe what they do and what their day is like. Get to know your future administrators. A principal that supports you can really help move things along when you need help.
Be prepared for backlash. I got the head spot when my HD was promoted. Even though the kids all knew me, I still got attitude from some (especially upper classmen) because I didn't do things the way he did.
My first year as head was rough. But each year afterwards got easier as everyone learned and adapted to the way I did things. I wasn't always right, but I tried to be approachable and open-minded.
This can be your hardest gig with so much to do, but it can also be your most rewarding. I wish you all the best!
p.s. Make sure to make some time for yourself. Your off time is YOURS. Don't let the gig consume you.