What grade (if ever) to switch 529 to conservative investments?

My kids are soon to be starting high school and the target investment strategies my 529s use are soon to be converting to more conservative funds. I am hesitant move the money and miss out on more years of potential gains. I understand the market could go in the other direction and I could lose money. I have about $150,000 for each child in the 529 accounts. I have the means to pay any and all future tuition without needing the 529 so I am not afraid of dips in the market requiring me to pull money at bad times. I would like to use the 529s though for the tax savings and to use the other money for other things. Should I just keep the investment strategy fairly aggressive or just follow the target investment strategy? Anyone just keep investing throughout the time their kids are in high school and college? I figure I can always use the funds towards potential grad school, kid's roth IRAs, or transfer the money to grandkids if I have money left after my kids finish college and any potential post graduate school.