Stable mutagen should change names gradually.

[Clarks]

If stable morphed your name in more than one steps, you would temporarily be Spigot the Farmer before fully becoming Spigot the Bear.
Using less mutagen than the necessary amount could leave you with an half transformed name. Or with some luck you could use this to eate a whole clan of Farmers.
Some guy wanting to ditch his identity could take blood from two or three people and use a bit of each in turn to get an entirely new name.