unhappy with eddy need a change lol

how early is dangerous for a trans to shift? i have a transgo shift kit not sure witch one nothing to hi-po but it shifts nice and firm like at 10-20 then 30mph with easy driving but any lower and 3rd is pretty spongy. if i shorten it to get wot will lower shift point hurt the trans. its a 904 rebuilt around 11000 miles ago. thanks