if you add a volume of water @ 150ppm,.... then add another volume with its own ppm of 550... neither will be the same once in teh substrate together. e.g. if 1L each, then you'd have half the ppm of each for a total of 350ppm (75 + 275).
you should be able to add cal mag to a fully dissolved solution of your "550ppm" without causing a precipitate.
unless it is a drastic change, i'd just irrigate as normal with this mix. skip the flush. Ah, i see you are in hydro, so simply changing the rez or bucket etc will effectivley transition to your new mix of nutes. easy-peasy.
Okay, i see the damage in the diary. the pic attached to question above looks nice. Check roots for any slime or off-color not related to nutes.
Pic 17 looks like nitrogen def.
The other damage looks like potassium deficiency?
try bumping up to a 700-750 ppm mix. double check your pH in any rez with issues and the roots, of course. easy to rule that stuff out, first. you calculate your ppms? do it for each molecules.. compare below:
130 / 50 / 200 NPK and 100+/80+/110 Ca/Mg/S - some of the secondary stuff will depend on tap water a bit, and should have your trace elements in any 1 part of your "hydro" nute lineup. consider these ballpark figures... adjustments from here as need based on observing plants. based on ppm you have fed, i have a hard time seeing a toxicity. it would have to be pH or ratio of nutes imbalance of large proportion.