In a nutshell, you will be fine. Dont dive all the way into the CUI pool yet, just stick your toes in for now. So what does that mean? Do the 07 install, make a copy of your mnu/mns files, and then menuload the mnu/mns file into Acad. It just got converted for you. You are now the proud owner of a CUI file.
That being said, now you can type CUI and open the editor and begin looking around at whats new. Dont get overwhelmed at first, as this just leads to being pissed at what Autodesk did to such a simple concept.
Make some changes, try moving things around, and see what you can break. At some point, you will throw up your hands and decide you like the old way better. This is ok. You can decide what changes you want to make, and go edit your mnu/mns. Delete the CUI file that was created, and reload your mnu. At some point you will have learned enough that you will get comfortable in the CUI editor. It took me 3 years and a lot of cussing and complaining