The manufacturer is currently out of 9K air handlers and has asked us to replace them with 12 Ks.
A 12K air handler can act as a 9K air handler and not affect performance. This is because the system contains a DC inverter that allows the system to provide the exact amount of energy required for a zone. Unlike old-fashioned systems that used to shut off, these modern systems adjust their output rates accordingly.
There is not much difference in cost – and you are getting a slightly larger capacity.