I've flipped at least two letters over time, and at least one letter on next-morning retest. J/P in particular has never seemed clear or significant to me.

I think this is actually the most common experience. The data I've seen says that scores on all four 'axes' are normally distributed. So it's actually fairly exceptional to have strong, predictable positions for every letter; cutting a bunch of normal distributions down the middle means most of us are on a boundary.

And yes, it's probably those silly E folks saying they never change. God forbid we wake up tomorrow testing as E, we'll have a paradox on our hands.