Some patterns are very difficult or even impossible to decrease and stay in pattern. I tried on a very small sample to do a double decrease between yarn overs but that brings the one pillar down to one stitch. You really need to get rid of all of them and I don't know how to do that. You could do a pillar and then do a double decrease in the next one, then do 2 pillars and another double decrease. This would allow you to carry the pillars a little further up (but the one stitch between pillars pulls them together and they don't look the same as they did), but it seems that you may need to ditch them eventually to keep decreasing. Then maybe you could keep doing that every couple of rows, but I haven't tried it to see what happens.
Another thing I tried is to change the pillars to mini cables, but you lose the open work look anyway. You could do something like that though and do a mini cable and then maybe rev st st over one pillar and do another mini cable. Then decrease away the rev st st so that the mini cables came together at the top. But you may need to eliminate more cables to get it small enough at the top.
The easiest thing to do would be to just drop the openwork and do a rev st st, st st or garter top on the hat. I often do some pattern or colorwork and then drop it at the top of the hat so I can do the decreases. It doesn't look bad, and may be the best option, all the other options will be a bit of experimentation to see if they work and may mean ripping back to try more than once.
I advise you put in a lifeline in the last round of the pillar work before you try your experiments. Good luck.