It kinda depends on how you are growing.
If you are using lots of grow medium.
like coco, or grow rocks,
that can accumulate salts,
it might be a good idea to flush with clearex,
between switching over to flower.
There are studies that have been conducted
on corn and other seed producing plants,
that show there is more than enough Nitrogen stored by the plant,
by the time it starts to flower,
that it does not need any additional N.
This has led to the lucas formula,
which eliminates "Grow" altogether,
thus greatly reducing the amount of N
the plant has available in the nute solution.
Here is what I do in my DWC and aero,
which are basically medium free.
I switch the lights,
and in a couple of days,
I flush the veg nutes
and replace them with bloom.
On the first day of 12/12,
I use a soaking foliar application of Penetrator/Reverse.
These phospho lipids send signals to the plant to start flower.
At 7 days, and 14 days, I repeat the Reverse foliar spray.
So I give them 3 total applications.
By now I am on 50% load of flower nutes.
With AN nutes, this is my max load.
With DM nutes I will go from 50% up to 80%
over over a three week period
and then start back down to 50%.
With both nute systems,
I stop feeding when the calyx' swell,
usually somewhere between week 8, 9, or 10.
Then I flush with plain water.
I use RO with no pH adjustment,
to force the plant use the N it has stored.
My RO only flush last from 1 week,
up to 2 months.
The purpose of the flush
is to allow the plant to use
the stored nutes it has in the roots and leaves.
There is no reason to adjust the pH of the water,
since the pH only affects the solutbility of the metals
that the plant can absorb.
We are not trying to add any more nutes to the plant,
so there is no need to adjust the pH,
and putting your probe into RO
with only some pH adjuster,
will cause you to need a new probe soon.
We want the plant to become deficient during the flush.