How to updated an encrypted 7zip archive without losing encryption

reading man openssl (especially the section PASS PHRASE ARGUMENTS):

Several commands accept password arguments, typically using -passin 
and -passout for input and output passwords respectively. These allow
the password to be obtained from a variety of sources. Both of these
options take a single argument whose format is described below. If no
password argument is given and a password is required then the user is
prompted to enter one: this will typically be read from the current
terminal with echoing turned off.

   pass:password
             the actual password is password. Since the password is visible
             to utilities (like 'ps' under Unix) this form
             should only be used where security is not important.

   env:var   obtain the password from the environment variable var. Since 
             the environment of other processes is visible on
             certain platforms (e.g. ps under certain Unix OSes)
             this option should be used with caution.

   file:pathname
             the first line of pathname is the password. If the same 
             pathname argument is supplied to -passin and -passout
             arguments then the first line will be used for the input 
             password and the next line for the output password.
             pathname need not refer to a regular file: it could for 
             example refer to a device or named pipe.

   fd:number read the password from the file descriptor number. This 
             can be used to send the data via a pipe for example.

   stdin     read the password from standard input.

openssl enc accepts -pass <arg> ... so, pick your arg from the list given above. eg:

 echo -n "secret" | openssl enc -aes-256-cbc -salt \
        -in file1 -out file1.enc \
        -pass stdin

I would assume that 7-Zip and other archiving tools compress before they encrypt, for the reasons stated in the linked blog post. But I was unable to find any documentation that confirms that, nor could I immediately ascertain it from looking at the 7-Zip source code.

However, I can explain why filenames aren't encrypted. As you might be aware, the 7z format contains a header with the file information and other metadata. 7-Zip will not encrypt this header unless you explicitly enable it. You can do this by checking the Encrypt file names box at the bottom of the Encryption segment of the archive creation screen on Windows, highlighted in red below.

On Linux and other Unix-like operating systems (and presumably the command line 7-Zip tool on Windows), you can enable header encryption by adding a -mhe=on switch to the 7z command.