You can upload the configuration from a ZD3k to a ZD5K but not the reverse. Make sure the ZD5K is running the same version of firmware as running on the ZD3K when the configuration was backed up. You can easily upgrade the 5K after importing the configuration. The new "Smart Redundancy" feature will only work with identical Zone Directors running the same level of firmware (9.0 and up) and with the same number of AP licenses. You can use the old "secondary Zone Director" IP setting on Access Points to seek out a second Zone Director if the primary does not respond. This form of AD hoc redundancy works as the Access Points seek out and request to join the Zone Director. But any users creation, DPSK's will not automatically get updated in the "back up" ZD, and the AP will reboot after connecting to and getting provisioned by the new Zone Director, dropping customer traffic. If using this AD Hoc redundancy feature make sure both ZD's are running the same level of code, otherwise the AP will get upgraded/downgraded if they change ZD's, and can cause long delays and AP's that flap between Zone Directors.
I hope it is helpful.