As long as oracle will be in the group dba, all will be ok, I think. If you just want to change the primary group, to dba, I see no problem. If you create a file or directory, it will have the users primary group, so you should know if that's what you want. To make sure everything is still accessable, you can keep oracle in the group orainstall.

On Solaris, I think you can just go on and edit the /etc/passwd and /etc/group. There is also some administration interface that you could use instead which's name I can't recall