What does "POS ITERV WAS CHGD" mean on the ACARS page of a Boeing 737?
Your aircraft is reporting its position periodically over the Aircraft Communications Addressing and Reporting System (ACARS) to the airline operations centre (AOC). 

Since there are costs related to sending these position messages, the interval in which these message are sent drives operating costs for the airline. 

This patent from China Southern Airlines (original Chinese version here) describes the behaviour you have observed on the MCDU. Apparently it allows the AOC to change the position reporting interval over ACARS remotely. If the interval is changed, a "POS ITERV WAS CHGD" text is displayed on the MCDU scratch pad. 

The relevant text from the patent (including translation &amp; typesetting errors): 


  ...
  
  [0087] PTIC message display unit, see FIG. 6, the module receives PTIC
  PTIC uplink triggering instruction, parsing PTIC variable, position
  determined by the adjusted multicast time interval T3, if the variable
  is valid PTIC, is given in M⑶U unit tip text message to inform the
  crew of the current position of the broadcast time interval has been
  changed, the present invention uses "P〇S ITERV wAS CHGD" text gives
  the unit prompts; the message is always displayed in MCUD the scratch
  area (Scratchpad) until a new the message will disappear after replace
  the unit or manually pressing CLR keyboard. ...