You could try using or adapting the method in this thread: [Only registered and activated users can see links. ], but be aware that you may cause grief to your plotter if it all goes wrong. I would think your internal memory size is maybe 4GiB as your OEM loaded chart and basemap combined size must be quite large.

I really do think your safest way is to use 'unofficial' charts on media cards and just leave the pre-loaded one alone if you don't wish to update it from Garmin.