This is a loophole that exists, yes.

10b5-1 plans set up by the company will sometimes have a rule that no changes are allowed for 30 days, which solves the problem. Not sure how widespread this practice is.

There are rules about amending your plans. My company has trading windows (a month or two after earnings, window open for around three weeks) for when people above a certain level are allowed to make trades. Even higher level employees must have a trading plan. You can only modify your trading plan during the trading window and there are restrictions and limits on how you can modify it and if you need permission to modify it.

Or something along those lines, I'm not high enough level to have to deal with the specifics so I only know the generalities I picked up while trying to determine if I needed to care.