Is it possible to replace already laid concrete with the new refined concrete using this mod? I have gave it a try and don't seem to be having any luck, thanks.

I came here to ask exactly this

If you're feeling brave:

1. Unzip the mod
2. Modify control.lua thusly:

replace

if not tile.hidden_tile or (not string.find(tile.name, "concrete") and not settings.global["ignore placed tiles"].value) then

with

if not tile.hidden_tile or (not string.find(tile.name, "concrete") and not settings.global["ignore placed tiles"].value and not string.find(title.name, "refined-concrete")) then

Should be around line 65 or so.

I have no idea how this will affect the patterning that it usually supports, but I don't use that anyway. Additionally you may need to add a new Roboport (or remove/replace) to get it to kick in. Simply making the change then reloading doesn't seem to trigger it. But once I dropped some new 'Ports it started laying refined concrete.