Valve is creating a new add-on campaign called The Sacrifice. The campaign is being released for both Left 4 Dead 1 and Left 4 Dead 2. Whether or not the actual maps will be identical or not is unknown at the moment.
The Sacrifice in Left 4 Dead 1 will feature the new campaign, but none of the features of Left 4 Dead 2. Melee weapons, new guns, new infected, etc. will NOT be added to the L4D1 version of the campaign. It's simply a new set of maps to play through.
The Left 4 Dead 2 version will still feature the same Left 4 Dead 1 survivors (Bill, Zoey, Francis, Louis), but will have all the features of Left 4 Dead 2 as well, including the new weapons and infected. It will basically be like a new set of maps for Left 4 Dead 2, as well as the L4D1 survivors being ported.
In both versions of The Sacrifice, players will be able to choose which of the four survivors sacrifices him/herself at the end of the campaign. The canon choice is Bill, but players will be able to choose any of the four when they play through the campaign.
In addition, the No Mercy campaign is also being ported to Left 4 Dead 2. L4D2's No Mercy will have the original Left 4 Dead 1 survivors, as well as all the new weapons and new infected of L4D2.
Valve is also making a 150-page digital comic to be released with The Sacrifice giving background on the L4D1 survivor's lives before and during the infection, as well as the final canon sacrifice by Bill before The Passing.No Mercy? Playing as Bill again?! Tuesday will not come soon enough!