Replace last comma in string of text

Hello,
I'd like to write a formula that will replace the last comma in a string of text with the word "and" but only if there are two or more commas in the referenced cell.

For example, a cell with the text "technology, health care, communications" would become "technology, health care and communucations". A cell with "technology and healthcare" would stay "technology and healthcare".

I have the following formula currently to replace the last comma, but I don't know how to add an argument that will tell the formula to replace the comma only if there are more than one: =IFERROR(SUBSTITUTE($L15,",",", and",LEN($L15)-LEN(SUBSTITUTE($L15,",",""))),$L15)

Any suggestions?