You seem to be describing Tags in your files metadata that you want to change in bulk. I call doing this kind of thing "auditing my library". This is where you sweep through the library and canonicalize everything to be consistent.
The general approach is to search (or sort) your library for the "bad tags", then highlight the files that contain these tags. Finally, you change the bad tags to good tags in one operation. For example, if you highlighted every file that contains "Mozart, Wolfgang" and then changed it to "Mozart"... all you have to do is put Mozart in the (Composer, Artist, etc) tag one time and it changes it for ALL of the files all at once.
Doing this by searching isn't bad if you have a half dozen to do. If you have more, I like to use a Panes view. Panes are very powerful and they have the advantage of each PANE showing you every single value in the entire library. For example, if you make a Panes View and have one pane that shows Composer, you can scroll through that list and see every value. When you see wrong values, you can select it, select the files, and then change that value in the Tagging Window to the left. All at once, you will have changed the bad tag value to your good one for those files.
I have made several different kinds of Library Auditing Views using Panes. This is actually a really cool thing about MC. It has allowed me to really take control of my library and to make all the metadata "perfect" relatively quickly.
Note that while in a Panes view, the search box at the upper right still works. So you can do a search for something like "Mozart" and then see all of the values in the Composer Panes area that contain that value. This is a very powerful way of combining the "show me all values" part of Panes with searching to narrow things down.
Best of luck.
Brian.