I have both a Nuvi 885T and an 855.
The main difference between these two units is the 885T is Bluetooth functional with a cellphone.
Both units have their latest version firmware version installed which is version 4.7
I may be wrong but I think boh of these units have the same hardware/PCB inside.
Both units have a remote button to activate the Voice Recognition feature.
I think the remote buttons are probably bluetooth devices.

if all the hardware is there it should be possible to turn the 855 into an 885T

From the diagnostic screen i get this info:

Nuvi 885T
PCB P/N NA
PCB Version 9
Unit ESN/Unit ID (unit specific ID number)
Flash 1 035344535430344780402b439600823f
Flash 2 NA
Voice Count 50

Nuvi 855
PCB P/N NA
PCB Version 9
Unit ESN/Unit ID (unit specific ID number)
Flash 1 035344535430344780409d8eeb00933f
Flash 2 NA
Voice Count 50

If I download the latest version of the latest 4.7 firmware and extract them the each have a file named: B072600.update
So can I further assume from this that the hardware on both units is the same.

I am new at this so if my suspicions are correct can I just flash the 855 unit with the 885T firmware and turn it into an 885T ?