The thing with poultry nipples - some birds get it, some birds don't - a bird that gets it, knows how to work the valve to get a nice, long drink. The ones that don't get it, or at least not as well, tend to stab at it, like they're trying to kill it! But I feel even they still get enough to drink.
But I wouldn't take away their old system during a hot summer. Go ahead & install the nipple system, and really work with the birds - especially the young ones - to help them figure it out. I've heard a rubber band works, where you leave the valve dripping a little, so they really get to see the water coming out of it. Once 1 or 2 birds figure it out, the others will learn by watching them. (Mine learned from the baby chicks, once they were integrated into the main flock.)
Keep in mind there are different nipple systems - vertical, horizontal, and cups with levers - if one doesn't work for you, consider trying again with a different style. And then find the right sized plastic bottle - coke bottle, juice bottle, mini water bottle, whatever fits inside your fountain - and fill it with water & freeze it. Plop in a fresh frozen bottle every morning, for cooler water for your birds.