In the 70's they de-emphasized the word ministry in favor of the word service. Our Kingdom Ministry became Our Kingdom Service. Prior to this time all JW's were said to be "ordained ministers."
Yes, 'ministry' became a dirty word for some reason, and did not appear in any of the publications as far as I can remember, the word 'service' being used instead. I remember Our Kingdom Ministry being changed to Our Kingdom Service and wondering why. There was no proper explanation in my opinion and I found it all rather irritating. I was pioneering at that time and I remember being verbally corrected whenever I said I was going out "on the ministry" someone would say "don't you mean you are going out in field service?" Then, as we know, it all reverted back again.