I'm not sure how much help this will be.I can give the when but not the why for my branch.
1 Michael Tousley aka: Michael Towsley b: Abt. 1635 d: November 3, 1712 . 2 John Tousley b: September 17, 1694 in Suffield, Hartford County, Connecticut d: December 24, 1764 in Suffield, Hartford County, Connecticut .... 3 John Towsley b: April 24, 1721 in Suffield, Hartford County, Connecticut ....... 4 Moses Townsley b: March 11, 1756 in Suffield, Hartford County, Connecticut d: October 6, 1803 in Chesterfield, Hampshire County, Massachusetts Burial: Huntsburg Center Cemetery, Huntsburg Township, Geauga County, Ohio .......... 5 Abigail Townsley b: October 30, 1791 in Westhampton, Hampshire County, Massachusetts d: October 3, 1859 in Geauga County, Ohio Burial: Fowler's Mill Cemetery, Munson Township, Geauga County, Ohio
The change to Townsley occurred after my line moved to Hampshire County, Massachusetts.Prior to that Tousley and Towsley appear to have been used interchangeably.Possibly a preference of pronunciation had something to do with it.This is only a guess but I would pronounce Tousley to sound like house and Towsley to sound like Towzley.
The change to Townsley with the insertion of the 'n' was probably a natural progression.I can't tell you how many times I've been called Mr. Justin.I haven't the foggiest idea where the 'n' comes from.