Dear Karen: There seems to be no rhyme or reason for the spelling of LAHEY and all it variants. for instance, in the Hearth Money Rolls for Tipperary in 1666, the following spellings occur: Lahy, Lahye, Laghy, Laughy, Laghie, Laghee, Laghey, Lahe, Lahey, Leahy, Lehy, Lihie.
in my home parish in Tipperary,these spelling occur, often for the same people: lihie, Lahy, Lihy, Lehy, Leehy, Leahy.
When my people came to canada in the 1820s and '30s, the name was spelled Lahy, Lachie, Laghy, Layhey, Layhe, and Lahaie,(the french variant, and LAHEY, which stuck.
When Griffiths did his valuation in the 1850s, he decided to spell all of these names as Leahy. Nowadays in Ireland nearly all the names of this surname are spelled Leahy.
As for the change from Leahy to Leahey in your family around about 1900,I suspect it has no significance at all beyond an aberrant spelling that stuck. Probably if you go back far enough you will find several other spellings as well.
Take care, dale