First, you don't have a SIM card in your GPS. SIM cards are for phones, you have a micro SD card, which is something completely different.

If you want to be completely certain you don't lose data on your GPS, you need to back it up. Create a folder on your computer and copy the entire contents of the SD card to it. Then make another folder and copy the entire contents of your GPS internal memory. This will let you restore if you have a problem.
Garmin's old software, Mapsource, did actually delete an old map and replace it with a new one. It always created a file named gmapsupp.img, so if you already had a file with that name then it would be deleted. The simplest solution for this is to change the name of the map file after you send it. In other words, open the Garmin folder and change gmapsupp.img to topo.img (or whatever you like). Mapsource was discontinued a number of years ago, but it still works.
Garmin's newer software is Basecamp and there is a companion program called MapInstall that does the actual installation of maps. It is designed not to delete existing files. But unfortunately it's complicated now. The current version of MapInstall is broken and cannot install a usable map on your GPS.
There are many threads about this problem here. And we just learned that Garmin has no intention of fixing this problem and they are no longer updating Basecamp/Mapinstall. This thread has more information and suggestions for how to deal with the problem. But it's a bad situation all around.
https://forums.gpsfiledepot.com/index.php?topic=4539.0