Replace date pattern

Question

I have a list, such as the following:

li = {"2017-10-16", 10.24, 10.71, 10.18, 10.61, 10.61, 16802200}

Extracting the first element li[[1]] I obtain 2017-10-16. My objective is to change the sign - to a comma ,. I tried using the Replace function, however my implementation:

Replace[li[[1]], "-" -> ","]

returned 2017-10-16.

How can I change - to , in the date listed above?