You can do this fairly easily, using a combination of sorting, playlists, smart playlists, and the multi-item Info window. I'll describe a few examples of how, using your examples of what...
make the composer be the artist, not the performer, e.g., Bach instead of Glenn Gould
Create a new playlist to use for manipulation of song information. For your example, add all songs that you want to have Artist = Bach to this playlist. If there is a simple rule, such as Composer = Bach for these songs currently, you can also create a smart playlist. To create playlists and smart playlists, click the Plus sign at the bottom of the Playlists sidebar in iTunes.
For this smart playlist, set it to Match all of the following rules:
Media Kind - is - Music
Composer - contains - Bach
This smart playlist shows all songs where the Composer field has the word Bach. You can also have a regular playlist for the songs that do not have an entry for Composer, that you populate manually with all other songs that you consider to be Bach. These two playlists combined now have all songs that are Bach. In each playlist, set it to use the Songs view, to show a plain list with columns. The view control is at the right end of the horizontal bar (set it to Songs). Show the useful columns for this purpose, such as the Composer column. Right-click the heading row of song list to show/hide columns. You can move the columns left or right by dragging column from its heading, and adjust the width for each column.
In each playlist, click on any song (so that you are IN the playlist) and do a Select All (Cmd-A on keyboard OR Edit -> Select All from menu bar). Right-click on the selection and Get Info. This shows the multi-item Info window for your selection. Change the Artist entry to Bach.
The above procedure is useful for a large number of songs that fit a particular criteria. You can Edit Smart Playlist to change the rules of that smart playlist to gather other large groups of songs automatically, and quickly manipulate them all at the same time. You can use that regular playlist again, and manually populate it with a new set of songs that have something in common (to change their song info at the same time).
For smaller groupings of songs, such as a particular artist or an album, you can make the changes directly in the music library song list (set to show the Songs view with appropriate columns visible like with the playlists). For example, sort the song list by the Artist column. For each Artist, such as "Bob Dylan," select all of the songs at the same time. With all Bob Dylan songs selected, right-click selection and Get Info. In the multi-item Info window, change the Artist entry from Bob Dylan to just Dylan. Repeat for other artists as desired. You can do the same thing with Album names, by sorting the song list using the Album column.
Some tips for manipulating the song list:
To select songs that are together on the list at the same time, click the first some song, hold down Shift key, and click last song. The two clicked songs and all songs in between are selected at once. For example, do this to select all songs on an album (or all songs by an artist) at the same time.
To select songs that are not together on the list, hold down the Option and Command keys, and click the songs you want to select. All clicked songs remain selected. To unselect a song, while holding down Option and Command keys, click a selected song.
You can do these actions in combination. For example, you can select all songs by an artist that are together on the song list, then select other songs separately, and then unselect some songs. This makes it easy to do complex multi-item Info window changes.
To sort the list of songs, click on the heading for the column you want to sort by. Click it again, to reverse the order. Some columns, like the Album column have several sort other sorting option.
Use a playlist and/or smart playlist if the number of songs to be manipulated is large, so that they stay together while completing the manipulations (instead of being "lost" in your library song list after making a change).
If you want to preserve information that you are changing, you can add it to the Description field.
