This thread's method will only work to change the basemap to Atlantic version for such devices as GPSMAP278 which are 'non-MSM' devices with the basemap stored in rgn 3 [hex 03]. The thread kindly linked above by windsurf is specific for loading a new detail map to rgn49 using WebUpdater.
As you posted followed my Post #9 regarding change to Pacific basemap, perhaps you're wanting to change the basemap to Pacific or Americas or even a small World version, if so use the method described in [Only registered and activated users can see links. ]. Several basemaps are available as pre-made RGN files in that thread also. That method can also be used for loading a primary detail map to rgn49 [hex 31], a secondary detail map to rgn10 [hex 0A] or a small OEM detail map to rgn50 [hex 32].