Harmonix has said transferring songs from Rock Band 1 to Rock Band 2 will cost you. Money.
When the sequel is released in the US this September, Rock Band 1 will get an update to add an export feature to the menu, according to MTV Multiplayer.
Upon selecting that option you will be prompted to pop in a password, which will be supplied with copies of Rock Band 2. Then you will pay; Harmonix said this will cost USD 5 maximum.
Once everything is paid for, the Rock Band 1 songs will sit on you hard drive and work in Rock Band. So far the feature has only been explained for Xbox 360.
Rock Band 2 is yet to be given a European release date. Rock Band 1 is available on Xbox 360, and will launch on PS3, PS2 and Wii later this year.