Did you ever find out why your Fairfax family changed their name to Harris?
My John Fairfax was the son of Joseph Fairfax (born c1794) in Nuneaton, Warwickshire
I have not yet found a baptism for Joseph, but have located an Parish Apprenticeship record, which I believe to be his and this gives his mother's name as Hannah. I also have his Army Attestation and Discharge papers.
My most recent find was a Hannah Fairfax and a John Harris, who had Banns read at St Nicholas, Nuneaton, Warwickshire in April 1793, but no record of a marriage has been found.
I thought you may be interested in this Fairfax/Harris connection.