I don't think the location codes for aerodromes changed as early as 1988. (It might have for international flights...).
For example, the
Autumn 1990 Aviation Safety Digest quiz question 7 opens with: "You are planning a flight to Melbourne (AMML) with an ETA of 0500UTC."
I think the changes (at least for pilots filling out flight plans and getting forecasts and NOTAMS through the system) changed in around December 91?