No, if they had of absorbed the nutrients they would have pissed off waste in the water. there by keeping some color to the rez. A TDS meter will tell you only how much ppm's are in the water. not what is piss and what is nutes and trace minerals. So we change our rez when we think nutes are gone.

how are the roots ?
If concerned or in doubt Always change the rez water out.
I would have changed and went to full strength with a meter to check.

p.s. Do you have a meter ?
Sometimes we mix things by the manufactures instuctions and it maybe low in reality.

Seems 400 is very low to ours. veg is 850 to 1200 and flower is 1000 to 1500. depending on the strain.