Hi.
I would have to point you to the compatibility matrix. There are usually no issues with downgrading a system, but this depends on if the system is supported by the old software. Newer software fix older caveats.
Some TP systems have requirements on what version of CUCM/CTS-MAN/CTMS/firmware that is compatible with the current hardware revision.
An example for this would be that if you have a system with GEN4-screen you would have to use 1.8.2 software firmware driver in order to make it work correctly.
My general advice would be to use the newest available software found on cisco.com for your product.
Thanks
/Marius