Special By-Laws can be retrospectively applied to the extent that they apply equally to all then existing and future residents of a Strata Plan, but not to the extent that they deleteriously affect any prior decision of an Owners Corporation or of an Adjudicator with regard to individuals.
It seems that your Owners Corporation wants to rule a line under your installation in order to prevent anything similar in future, and whilst that’s its prerogative, if it suits you to get moving with your aircon. installation sooner rather than later, that may avoid further discussions / delays.