My father's paternal haplogroup recently changed on 23andMe. I sent them a question about this and received the following reply:
...an update we released to the paternal haplogroup algorithm on July 30th, which considers an expanded set of variants on the Y chromosome. As a result, certain customers on version 5 of the genotyping chip have observed updated assignments — most often more precise ones. If your paternal haplogroup assignment changed, you can learn more by visiting the interactive tree of all paternal haplogroups in the Scientific Details of the paternal haplogroup report.
Lastly, 23andMe references the ISOGG 2016 reference tree for our paternal haplogroup assignments.
So, if you have a v5 test (Aug 2017 and later), you might want to check your paternal haplogroup assignment to see if it's been updated.