I tried and I can confirm that it sounds too good to be true.
I decided to replace only Switzerland data for a start
First POI I replaced poi completely and unit rejected it:
Spoiler: too large pic 1
I renamed map to xxx_old and copied new data but somehow unit found renamed map and loaded it:
Spoiler: too large pic 2
File structure on SD card is following:
-device.nng
-iGO
--license
---FTEN_14CY_EU_SUB<at>13Q3.lyc
--content
---global_cfg
---lang
---map
---phoneme
---poi
---scheme
---voice
---vr
---vr_vde
I guess somehow each file hash is stored in license file but I would appreciate any help :)