Hi there,
You really don't have many options here as to whether or not
to upgrade both Unity and CUCM (Communications manager)
Both of the versions you are running have been EoL'd (end of life)
by Cisco so even if you didn't want to upgrade, you're sort of forced
if you want Cisco support.
On the Unity front, even the newest 8.0 version is now announced as EoL
with no future versions to be released so you would need to look at moving
towards Unity Connection if you want to stay on the Cisco platform.
End-of-Sale and End-of-Life Announcement for the Cisco Unity Version 5.x
http://www.cisco.com/en/US/prod/collateral/voicesw/ps6789/ps5745/ps2237/end_of_life_notice_c51-606617.html
End-of-Sale and End-of-Life Announcement for the Cisco Unified Communications Manager Version 6.1
http://www.cisco.com/en/US/prod/voicesw/ps6788/vcallcon/ps556/end_of_life_notice_c51-589088.html
Of course, with upgrades comes the need for new hardware in most cases, so an eye
towards virtualization is the way to go
Unified Communications in a Virtualized Environment
http://docwiki.cisco.com/wiki/Unified_Communications_in_a_Virtualized_Environment
This older blog From super NetPro Hailey is a nice place to start the Unity to
Unity Connection discussion;
http://www.netcraftsmen.net/blogs/migrating-to-cisco-unity-connection.html?blogger=David+Hailey
Cheers!
Rob
PS: I didn't want to even try to get into the enhancements between these versions
as they are pretty vast
"And if I should fall behind
Wait for me" - Springsteen