Welcome to MT, Alan, and welcome back to training!
Any advice for a former Black Belt who will start as a White Belt?
My honest opinion, Alan, is that there is no such thing as a "former Black Belt." Take a look at what you put in your profile here - - "Tae Kwon Do, black belt." Color belts are always changing as you progress and learn; a Black Belt is a rank for life. If your rank is in Taekwondo, and you are going back to Taekwondo training (regardless if it is the same teacher, school, organization or not), your previous rank is where you should start, in my opinion.
I have been at this teaching thing for a long time. I have known many Korean Grandmasters who will simply leave it up to the student to choose what rank they will re-enter training after a long absence. I have known some who will have you demonstrate what you remember, then rank you where they think you should be.
If you came to my school, and showed me a legitimate certificate of rank for a 1st Dan in Taekwondo, that is where I would place you. It does not matter to me what other students think (my students respect my decisions), and I am not concerned with what you might have forgotten, or how rusty you are. You only have to learn how to ride a bike once, or learn to swim once, then that skill is released and understood for life. It does not matter to me how long you have to stay at your 1st Dan rank until you are caught up to test for the next degree - - there is no rush. However, you earned that rank once, and I don't see any reason you should not be permitted to wear the belt in training, even if you were 80 or 90 years old.
If you were certified in a completely different discipline (say Judo or Aikido), and starting up again in a new discipline like Taekwondo, then there might be a reason for beginning at a color belt rank (not necessarily white belt), and even then, you should be able to advance at a smoother pace than the first time through.
If starting at a white belt in Taekwondo is an option given to you, and it is you who are choosing this rank, I would say you are being too hard on yourself, and placing too much emphasis on the belt color. Usually, it is the other way around, where people care too much about having a higher belt, but your doing the same thing in the opposite direction. You are being more concerned about the belt and the rank, than the fact that you earned the knowledge once, and now you are going to brush up on what you know. You will probably learn new things along the way, but so do all of us. Don't think you have to wear a white belt, in order to learn what the white belts are doing. Don't think that your skill has to be as good as any other Black Belt in order for you to justify wearing the belt yourself.
Hint: Spouse is a practicing Black Belt and it feels a little weird.
If this "hint" means what I think it means, then this is something you will have to deal with in your own development. Again, your rank, or anyone else's rank should not be of concern. Wear the belt you earned, and focus on your skills as you train. My wife is a 3rd Dan, and I am a 6th Dan. If it were the other way around, I might have the same concerns you are feeling, but I would have to learn how to deal with that myself. The mere mention of this difference between your rank and your wife's, reveals that you are focusing too much on rank, even though you have made the humble effort of starting at white belt (latent ego).
My advice: Be the Black Belt that you are. Wear the Black Belt and spend 2 or 3 years finding out what that really means to you. In the mean time, enjoy the fact that you are alive, and healthy enough to train in the Martial Art - - and don't be so concerned about this rank thing!
Good luck!
Chief Master D. J. Eisenhart