We put this question to Shaykh ‘Abd al-Rahmaan ibn Jibreen (may Allaah preserve him) and he answered as follows:
It is permissible to do that if they are no longer working or if they need to be repaired. If it is possible to fix them and keep them, then that should be done, otherwise they may be sold and the money spent on new ones, even if the money will buy fewer of them. If there were ten old air conditioners and the money raised by selling them will only buy five new ones, then that is what should be done.
And Allaah knows best.