How to formalise an ad-hoc maintenance arrangement

I wrote some software for a family member's joint business a couple of years ago that is used constantly and very much a critical part of the daily operations of their business. Roll on to now and the family member has decided to resign (though retaining their stake), leaving it in the hands of the remaining owner and the investors. There was never much thought given to maintenance in terms of formalising the arrangement, there was the occasional issue with a third party integration that I would resolve without charge (because it was family), but it has been very little hassle. He understands how the software works at a process level and its quirks, so staff would never need to approach me directly.

Now that he will be gone I will potentially have to deal with staff training, direct support if there's an issue, angry managers, etc, currently with nothing in return for myself. If the software or integration fails, then the business grinds to a halt. How should I go around approaching this issue given that they are used to paying nothing, or would it be better for them to approach me when they first need my assistance?

I would want to make sure that I am correctly compensated with a monthly retainer. I wouldn't be able to provide anything in the way of an SLA because I have other work commitments. Any advice would be appreciated.