The Navigator Flight Controller has been released, if that’s what you’re talking about, and we’ve also made our remaining stock of Pixhawks available for purchase 
From an interfacing perspective, the primary differences between Pixhawk and Navigator are as Rusty described:
- Navigator requires Raspberry Pi 4B (all BlueROV2s to date have had 3Bs)
- Can be bought as part of the Navigator kit
- Navigator uses JST-GH connectors for the sensors and power module (per our connector standard), whereas Pixhawk uses DF13
- Our adapter board or I2C Bus Splitter can be used for interfacing with existing sensors that have a DF13 connector
- The Navigator kit comes with 3 6-pin JST-GH cables that can be used for the power module and/or serial connections
I believe we’re making an upgrade guide that walks through the process, but that may not be finalised until the upcoming BlueROV2 revision is released.
EDIT: The retrofit guide is available here
If you’re wanting to compare specs the main ones are presented in the Flight Controller Boards section of our Technical Reference
The upcoming BlueROV2 revision will use a locking enclosure, but won’t be changing its tube size or thruster sizes. As has been mentioned elsewhere the T500s wouldn’t fit in a BlueROV2 (and neither would a larger tube)