How to change the name of a vim buffer

You can use :file newname to change the buffer name.

From :help :file_f:

Sets the current file name to {name}. The optional ! avoids truncating the message, as with :file.
If the buffer did have a name, that name becomes the alternate-file name. An unlisted buffer is created to hold the old name.

Rename the current buffer with :file <new-name> (or :f <new-name>):

:f[ile][!] {name}

Sets the current file name to {name}. The optional ! avoids truncating the message, as with :file. If the buffer did have a name, that name becomes the alternate-file name. An unlisted buffer is created to hold the old name.

If the buffer already has a filename, :file will not change the filename and will only change the alternate filename. You'll need to clear the name of the buffer with :0f[ile] to be able to put one.

:e foo

:0f
:file bar