Background
With the latest firmware (145) update released by Philips for the Hue App and V2 bridge, one of my Bridges updated to 145 and one remained on 144 and was no longer accessible via the Hue App.
After discussing with Hue support they agreed to replace the Bridge but there is no way to either backup the set-up from the old Bridge or transfer to the new one. (There is a transfer ability from the V1 bridge to the V2 but not between two V2 bridges 
)
Process
I thought I would document my lessons learnt and overall approach for anyone else who experiences this same issue. 
First, create a local HE Backup HE!
1/ Access the HE App>Hue Bridge Integration and take note of all the Hue devices imported into HE from the broken HUE Bridge;
2/ Via the HE App>Devices rename all of the devices from step 1 by adding the word 'Old' to the end of the existing Hue device name. This will enable you to add the new Bridge and remove the possibility of duplicate names being added to HE;
3/ Next you will need to manually add all of the devices from step 1 back into the replacement Hue Bridge. I created an issue here because I returned the Bridge without first Resetting it, although it did mean that I could add each Hue device individually. 
If possible reset the old Bridge as this will unpair all Hue devices and make it easier to add them to the new Bridge.
4/ I used a combination of approaches here:
a/ With the old Bridge powered down, turn the power off and on again to your Hue devices. As they won't be able to connect to the Bridge, they will switch on when the power is turned back on. You can then use a Hue dimmer switch to reset each device by holding the dimmer switch within 8cm of the device and pressing the On and Off buttons at the same time, for about 10 secs. The device will start flashing, keep the two buttons pressed until the device stops flashing. The device is now reset and can be added to the new Bridge using the normal Add Light process within the Philips Hue App;
b/ Find the Hue serial number, (this is a 6 hex number, normally near the bar code), in the Hue App>Add Light select Add Serial Number to the search and this will first reset the device and then add it to the new Bridge.
c/ Hue devices like Dimmer Switches and Motion Sensors have a reset switch accessible via a small pinhole that can be held for 10secs with a straightened paperclip until the LED on the front of the device flashes.
5/ Once all the devices are added to the Hue App, open HE App>Hue Bridge Integration and Add the new Bridge and select the Devices you want to add;
6/ Now by opening each HE Device from step 1, scroll down and you can see all the HE objects that use the old devices. Edit each one (except your Voice Assistant linking), in turn, replacing the Device with Old at the end of the name with the new Hue device.
7/ If you use Alexa or Google Home integration, don't delete the old device or unlink in HE just yet! Open the HE App>Alexa or Google Home and link the new devices to your Voice Assistant. (If you delete or unlink the old device first, any Actions and Grouping will be lost) Then go into your Voice Assistant App and change any use of the Step 1 devices to use the new Hue device;
8/ Once completed unlink the old devices from HE App>Alexa or Google Home and then delete the Step 1 devices.
