I think the litmus test is weather they are backwards compatible with old maps/campaigns from the original engine/game.

Half-Life 2 sure won't play quake maps nor will it play hl1 maps.

> Half-Life 2 sure won't play quake maps nor will it play hl1 maps.

Not without modifications but Half-Life: Source is essentially a tech demo to show that they can be ported easily (if you are OK with dropping some pesky features like randomized wall textures).