I just got a new macbook pro. It runs Lion.
I have a time machine backups of my old snow leopard machine. If I use Migration assistant to move data to new machine via time machine backup, will it erase my Lion OS? I need Lion but I need old data.
No, you'll be fine. Migration assistant migrates user accounts, applications, setting and/or files (depending on what options you choose), but it won't overwrite your Lion operating system. See Apple's guide to using Migration Assistant here.