There's good news and bad news. The good news is that the next time you launch Team Fortress 2, you'll automatically start upgrading to "Steampipe," a new content delivery system which "improves the way content is downloaded, updated and stored" according to Valve. It means faster downloads, quicker boot times and map loads, and better installation and management of mods.
The bad news is that Valve predicts the download will take a while—even if you have a fast hard drive. "It might look like it's hung. But it's not. Don't interrupt it, just please be patient and let it think," Valve wrote in a recent blog post.
According to the SteamPipe FAQ, if you have custom game files—like those for custom HUDs, hitsounds, skins and such—will need to move files to a different folder:
Previously, each user had a folder that contained several game files.
For example, [steam installation directory]/steamapps/[username]/team fortress 2/.
When the game is converted to SteamPipe, the installation directory is shared between users. The directory will probably be something like
[steam installation directory]/steamapps/common/Team Fortress 2/.
NOTE: In this article, we will use folder names particular to Team Fortress 2 in the examples. The same principles apply to all of the games being converted, so just replace "tf" with "dod", "cstrike", or "hl2mp"