For instance if I Open a Assy or Part in Creo 3.0 and they are created in Creo 2.0, they have some funtions in status Obsolete. Even i can open them in Creo 3.0 but I see these issues. I would like to know if these issues can cause problems in the future.
Could you help me with this answer??
They won't cause you any problems. (unless you are talking something like a ProE version 9 file being opened in Creo 3, then maybe it will be an issue)
If it really bothers you, you can run Pro/Batch to open and save the files in the version of Creo you want. But I've never had a need to do this.