I'd say go with you common sense/gut feeling and use the meltagun points. However, just ask your opponent before a match or an official at a tournament. Replace, could be interpreted as merely swapping the weapon with no point change OR replacing the option of lascannon with the option of a meltagun points and all. However, I'm a big believer in common sense even if GW isn't, so I'd go with the meltagun price for the points.