The key difference is that the new image’s base OS is a ‘Stretch’ image (old one was ‘Jessie’ i.e. different versions of Raspbian/Debian).
To make the upgrade, you do need to reflash the SD Card - personally, I’d buy a new card so you have a path back if it is not successful.
You will need to backup the data and the import it. As you have upgraded to 9.9.4 that should be less of a hassle.