Very pretty pattern. To decrease for the crown of the hat you're going to have to give up either the lace pattern or the cable pattern first. It's really a matter of preference for you.
If you decide to maintain the cables as long as possible, then do the decreases every other round within the 5 lace sts. You'd have to change these 5 lace sts to either all purl or all knit sts. Eventually the decreases will eat into the cable pattern too but this will be further up the crown.
If you decide to give up the cable you could maintain knit sts over these 6sts as you decrease. Decrease as k2tog or ssk every 9sts on the first round (work 9sts, k2tog), then every 8sts on round 3, every 7 on round 5 etc. Eventually this will eat into the lace pattern and you'll have to go to knit sts here too.