Hi Deepak,
I recently upgraded couple of 5505s from 8.4->9.x during some testing purpose and did not notice any major changes. Couple things (did not not impact any of my testing though)...
1. It adds some udp/tcp deny rules in the beginning of configuration.
2. Ip v4 and v6 terms, you can specifically use (instead of 'ip')
As every environment is different, I suggest you check the release notes. Hardware- same memory as 8.4.
hth
MS