If you are painting over a semi-gloss with ordinary matt, then it is best to undercoat the walls first. This will ensure that the paint sticks properly, if you don't then the paint may not stick correctly and it "may" eventually peel off in sheets.
If you don't want to use an undercoat, then as an alternative, you should sand the walls with some fine sandpaper (180 grade). Then dust down the walls before you apply the emulsion. This will deaden the gloss in the paint and give the walls a bit of texture that will allow the matt paint to stick to it better.