Be sure you understand the basics before you attempt any such cross-flashing. The method used in Post #79 utilizes a HYBRID firmware, comprised of the boot.bin from HWID 0827 fw Version 7.80 (the latest) and the remaining components from HWID 0943 (nuvi 465T) Version 4.20, also the latest for that device. They've been blended together using RGN_Tool and the HWID of all the components overwritten to 0827. Maintaining the original HWID of the recipient device lessens the change of bricking it and in the unlikely event it is bricked using that kit it makes it much easier to return to full nuvi 2x5W. Conversly, if the FULL donor firmware is used and the HWID of the device changed to 0943 then the chance of bricking increases and the opportunity to return to 0827 is greatly lessened.

Your 255W has very old firmware, in fact it's probably one that was loaded in the factory. V3.21 doesn't show in the change log at all. V2.80 was superseded by V4.40 in the official history:
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Perhaps V3.21 was issued but then quickly withdrawn because it was buggy. In any case you should update it to V7.80 before attempting a cross-flash to nuvi 465T.

So what exactly do you mean when you say this: "Nothing changes with the Garmin Express upgrade!" ?? ... Do you mean it's not recognized by GE, or it fails during the process, or it seems to load current fw to the device but doesn't update afterwards? Note that you have to safely disconnect and let the device reboot to load new firmware after updating with GE.