The NSW Strata Schemes Management Act (1996) applies, where Sect 62 outlines the unfettered responsibility of an Owners Corporation (O/C) to maintain and repair its Common Property, and Sect 65A sets out the procedures whereby Owners (and the O/C) may make changes to that Common Property, and specifically at Clause 3 where the responsibility for ongoing maintenance and repair of any changed items, consented or otherwise, remains with the O/C unless a Registered Special By-Law states differently.
As the Secretary of a self-managed plan it pains me to say so, but your case is aided by the facts that it’s extremely difficult for an O/C to claim that it was unaware of unauthorised changes to its Common Property by a Lot Owner, and to then insist that a subsequent Owner of that same Lot is responsible for those changes, and additionally because downlights actually extend into the Common Property (roof space) the extent of O/C’s liability for maintenance and repair is strengthened.
I’d suggest that you strive for some form of compromise such as sharing the costs in some proportion, because if push turned to shove your O/C could resolve not to maintain the downlights, and instead to just remove them and restore the Common Property (ceiling) to its original state.