Has anyone done research on the use of 'Le' in place of 'Re' in Irish manuscripts. From what I've read, it appears that 'Re' is current until the early 18th cent. The earliest use of the form 'Le' that I have come across was in a document published in 1728. But I have seen 'Re' in another document from the 1740s.
The form 'Re' appears to have entirely disappeared by the end of the 1700s

Is it known when the changeover occured, and why ?

There are placenames using both forms Tóin le Gaoith and also Tóin re gaoith.

I think historically, re and le aren't the same word.

"Re" must come from Old Irish "fri" (which means "against"), which has become "ri" in Scottish.
"Le" comes from Old Irish "la".

They have merged and now only "le" is used, as far as I know.

_________________
Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne