man mount says:
Mount all filesystems (of the given types) mentioned in fstab (except for those whose line contains the noauto keyword). The filesystems are
mounted following their order in fstab.
Attempt to remount an already-mounted filesystem. This is commonly used to change the mount flags for a filesystem, especially to make a
readonly filesystem writable. It does not change device or mount point.
The remount functionality follows the standard way the mount command works with options from fstab. This means that the mount command only
doesn't read fstab (or mtab) when both the device and dir are specified.
mount -o remount,rw /dev/foo /dir
After this call all old mount options are replaced and arbitrary stuff from fstab (or mtab) is ignored, except the loop= option which is
internally generated and maintained by the mount command.
mount -o remount,rw /dir
After this call mount reads fstab and merges these options with the options from the command line (-o). If no mountpoint found in fstab than
remount with unspecified source is allowed.
So, that is the difference.
Now what should you use after modifying
/etc/fstab? Well, that depends. If you have a number of filesystems in
/etc/fstab that are unmounted,
mount -a will mount them all. That may not be what you want, but it may also be exactly what you want.
/dir is not yet mounted, you could simply
mount /dir, which will leave the rest of the filesystems mentioned in