To change Organizer you must disband the family group and form a new one. This can be problematic if a child account (that has a special meaning with Apple) is involved and the Organizer is unavailable. You would normally have a second Family group set up and the Organizr woukd arrange for transfer of the child to a new family group. You could probably do it by asking Apple to close the child account, or at worst abandon that account altogether. In any case you can leave the family but the child cannot. The child has to be transferred or their account deleted.
You could ask Apple what to do in this situation. I have not heard of them doing anything in these situations I have seen before. Refer to this page for Apple Support features ➔ Contact - Official Apple Support
Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.
This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple for support and service - Apple Support