BlueWolf72 over at ModDB, has interviewed Valve's own Rick Ellis.
Check out the full interview http://features.moddb.com/103Many mod designers are already planning on porting over their HL1 mods. Will this be a drastic change for the team involving a steep new learning curve or shall it be intuitive?
Rick Ellis: Of course this depends on the complexity of their mods, however in most cases it will be very easy to port a mod to Half-Life 2.