It may be what the planners are talking about is 'agricultural notification'. Basically if you want to put up an agricultural shed, you have to pay a fee (much less than a PP fee tho) and submit it as an agric prior notification. This means the planners then look at it and confirm that you are a holding and it is for agric purposes and they are happy with construction/drainage/position etc. After 30 days they will either have confirmed you can go ahead under the agric exemptions from planning, or will write to you to advise it doesnt come under the exemption and a full PP app is required, with their reasons.
If you dont hear from them at all within 30 days of them registering your prior notification, you can advise them in writing that they havent complied with the rules and you are going to go ahead. Or at least thats how it all works where we are. NB We have a 40 acre holding and i know the rules are tighter if you are under 5 hectares (ours is 17 point something) so there are probably extra restrictions and hurdles for your holding - check out what they are with the council.
I wouldnt advise going ahead without complying just cos there are other old sheds, the rules have changed since they went up. Its still a big benefit and much cheaper than PP and it means buildings that wouldnt have a cat in hells chance of getting PP if non agric can be put up in order to store feed/machinery/hay etc.
Ring them up and talk to them, find out if the above is what they are talking about. Its not change of use, but it is an admin hurdle.