Internal memory is not a card, it is simply internal flash memory.
When you first connect the unit to Home, Home asks what memory location you want, internal or external (sd) card. The location you select is where the installation of whatever you download is attempted.
Before any updating, make a complete backup of your unit's contents using Explorer, not Home. Ensure hidden folders/files are showing.
If you have stuff on a card, then you'll make two different backups: one for internal contents to one folder on your pc and a second backup of the sd card.
So, if you want the new map to go on the card, select the card in Home when you first start. The installation will then go on the card, and overwriting the old map's folder contents. Your custom poi's should still be there in the map folder after the update is completed but, if not, simply copy them back from your backed up map.