Is my tranny on its way out?

I'm new here, and could use some help. My duster has a 727 installed by previous owner, said it had a shift kit. It has a Lokar kickdown cable and shifts at appropriate rpm. My problem is, it shift nice and firm from first to second, but feels soft from second to third(seems to be worse under heavy throttle). I changed the fluid and filter but didn't help. Could the bands need adjusting or does it need a rebuild?