The small base brushes look like mine. My first Sonicare had the large base, screw on type brushes; it died after several years of use by myself and my Kentucky born and raised lovely wife (she has all of her teeth, shoes and got some larnin').
I've seen only two different styles. One has a big base on it versus the other. As long as you stay within these styles I think they will work accordingly. But I certainly am no expert on Sonicare replacement heads.