There is another piece here, that can be made worse by working for the practice buyers. Some may think they can continue to "request" (i.e., thinly veiled demand) you to work on their returns. I've had several of those instances since I sold of 3/4 of my practice about 7 years ago now.
Two of those are going to be told (once their returns are filed for this year; both on on extension for reasons related to events outside their control) this year that for next year they either have to work with the guys who bought my practice, or go elsewhere. Not wanting to let go of me I can understand at some level, but I also want to have fewer, not more, clients. I've been letting it slide not wanting to be hard-nosed about it, but it's time.
Yet another issue is that a new owner/practitioner - even if you have trained them! - is going to do some things differently. Checklists instead of organizers. Letters sent out later in January, or only by email, or paper copies of returns only to those who ask. They won't take checks, or Amex, or want payment up front instead of upon delivery. That will feel "wrong" to the retiring practitioner. It's not wrong; it's just different. Don't bad-mouth them for it, don't try to change it, just back off and let them deal. It can be surprisingly hard at first!