I already posted my old game. When I post the remade version, should I make a new entry? or replace the older entry?

I guess could just include a link to the new game in the old game's page. This way people will play old  first and then the remake.

Any thoughts?

Create a new page since it is no longer the same game but part two.  Also (that's what I will do) post Images of your old game on the new game page to better see the difference additional to the link to the older games page.

Another thing you can do is pack up both executables in one .zip file and upload that to itch.io if it falls within the size limits!