They are compatible. That said they did fix plenty of bugs. Which means that any of the fixed features will work differently (better.)
There is a large amount of changes so I will just copy them here.
8x64
" Release Version: 8x64
±------------------------------+
| New Features and Improvements |
±------------------------------+
-
Added the ATAG command. (Aggregator Addressing Update)
-
Added the trace route command on unicasts.
-
Added NACK functionality on unicasts.
-
Added the ATFN command. (Find neighbor)
-
Added the test link cluster ID.
-
Added a number of diagnostic commands.
(See the Diagnostic sections available in the X-CTU Modem Configuration tab)
±----------+
| Bug Fixes |
±----------+
-
Querying ATOS for large sleep periods was resulting in the wrong value.
-
Certain payloads of encrypted packets were not getting sent correctly.
-
Fixed bug where a remote WR command would immediately apply changes to CH when not desired.
-
Fixed bug where changes to the IF parameter would apply a value other than that set.
-
Removed the BD8 option because the error margin at that baud rate is unacceptable."
8x65
"
INTRODUCTION
These release notes document changes made to the firmware on XBee and XBee-PRO
DigiMesh 2.4 radios.
ENHANCEMENTS
-
Added the ability to enable a clear channel assessment (CCA) feature with
the ATCA command. -
Added Test Link functionality for RF link reliability testing.
-
Added the ability to enable a Trace Route or NACK on transmitted packets.
-
Added a Find Neighbor (ATFN) command.
-
Added a command (ATAG) for building and repairing routes to a node and for
replacing aggregator nodes.
BUG FIXES
-
Fixed bug where ON_SLEEP didn’t always work correctly after power-up.
-
Querying ATOS for large sleep periods was resulting in the wrong value.
This has been fixed. -
Certain payloads of encrypted packets were not getting sent correctly.
This has been fixed. -
Fixed bug where a remote WR command would immediately apply changes to CH
when not desired. -
Fixed bug where changes to the IF parameter would apply a value other than
that set. -
Removed the BD8 option from the .mxi file because the error margin at that
baud rate is unacceptable. -
Fixed bug where some radios would not receive data after powerup until after
the receiving radio had transmitted data.
KNOWN LIMITATIONS
- When streaming broadcast messages, occasionally receiving radios will
output a packet out its uart multiple times."
As you can see they fixed and added quite a few things. I recommend updating all your modules to the new firmware it has worked well for me. Though I would also use all the same firmware if possible.