Here's a walkthrough for anyone looking to pop the achievements for the 20th Anniversary Update if you've already completed them in the original Half-Life 2: Episodes 1 & 2.
1: Search the Steam Store for the soundtrack Half-Life 2: Episode. This should display the two soundtracks for the original games.
2: Select an episode's soundtrack, then follow it to the original, now unlisted, release of the episode.
3: Select Play now. This should install the original version. Select Play now a second time to load the game up proper.
4: Quit from the main menu.
5: Repeat steps 2-4 with the other episode.
6: Load Half-Life 2.
By doing this, it will pop every achievement you've already completed in both episodes.
Credit:
https://steamcommunity.com/sharedfiles/filedetails/?id=33664...