I'll just quickly come back to this but briefly as I don't want to derail the OPs thread. I had a closed adoption but some dealings with the agency when I was starting a search for my BF. The person I dealt with had been there for years and years and remembered my adoption and had also dealt with my parents AND my birth mum. Speaking to her, I was conscious of a conflict of interest, I think that is the best way to put it. In fact, it is possible she lied to me a bit about details she had on file, I think with the goal of preserving my BM's privacy which is what was agreed with her at the time and where she, I think, saw her primary loyalty. However, it is also possible my BM gave false info. Not massive lies but just slightly. In any case, my dialogue with them improved when this person moved onto another role and I was dealing with a new contact there who had had no connection with my parents or my birth mother. She was able to speak more freely and seemed to have MY interests at heart.
In the case of the OPs DC, before transitioning the relationship with SW over to the DC, I would expect some sort of 'hand over' and discussion about how the relationship will work and that it was made clear the SW was there to support the DC now. In that regard, the relationship isn't 'like for like' however easier that would be administratively for the SW. The DC is still the child in this situation however old they are in reality, in my experience these feelings don't really go away. (I am 50.) So, I would expect that any dialogue would be approached with that sort of sensitivity in mind. Not just, well, here you go, we're going to pick up where we left off with your adoptive parents, 'because you're the adult now.' I would also want to understand whether the DC had dealings with SW in a way they might remember and whether any of those feelings are negative, for whatever reason.
I don't know what the legal rules are regarding privacy. Obviously they'll have to follow that. But I suspect the OP is in this situation because SW know and feel comfortable with that relationship.