I can’t say for sure on this, but I would assume that the system would only give a prompt to change the password for accounts which currently have less than 7 character passwords when the account is used to sign on.
I would suggest trying this when the machine is not being used (maybe over a weekend). Make sure your own account has more than 7 characters for the password. Sign on and <b>leave the session connected</b> for the rest of this. Make 2 test accounts – one with more than 7 characters password and one with less than seven. Use SAVSECDTA to get a backup of your security settings. Change the system value QPWDMINLEN to 7, try signing on on another workstation/session with both test profiles.
If it doesn’t work, you can always reset the QPWDMINLEN and if necessary restore the security data.
I looked at the help text when I did a DSPSYSVAL of QPWDMINLEN and it states
“A change to this system value takes effect the next time a password