I am supplementing chinmaya sarupriya's excellent answer.
Nirguna Brahman is inert. There is no question of it changing to Saguna Brahman.
A BRAHMO DEVOTEE: "Sir, has God forms or has He none?"
MASTER : "No one can say with finality that God is only 'this' and
nothing else. He is formless, and again He has forms. For the bhakta
He assumes forms. But He is formless for the jnani, that is, for him
who looks on the world as a mere dream. The bhakta feels that he is
one entity and the world another. Therefore God, reveals Himself to
him as a Person. But the jnani — the Vedantist, for instance — always
reasons, applying the process of 'Not this, not this'. Through this
discrimination he realizes, by his inner perception, that the ego and
the universe are both illusory, like a dream. Then the jnani realizes
Brahman in his own consciousness. He cannot describe what Brahman is.
"Do you know what I mean? Think of Brahman, Existence-Knowledge- Bliss
Absolute, as a shoreless ocean. Through the cooling influence, as it
were, of the bhakta's love, the water has frozen at places into blocks
of ice. In other words, God now and then assumes various forms for His
lovers and reveals Himself to them as a Person. But with the rising of
the sun of Knowledge, the blocks of ice melt. Then one doesn't feel
any more that God is a Person, nor does one see God's forms. What He
is cannot be described. Who will describe Him? He who would do so
disappears. He cannot find his 'I' any more.
"If one analyses oneself, one doesn't find any such thing as 'I'. Take
an onion, for instance. First of all you peel off the red outer skin;
then you find thick white skins. Peel these off one after the other,
and you won't find anything inside.
"In that state a man no longer finds the existence of his ego. And who
is there left to seek it? Who can describe how he feels in that state
— in his own Pure Consciousness — about the real nature of Brahman?
Once a salt doll went to measure the depth of the ocean. No sooner was
it in the water than it melted. Now who was to tell the depth?
"There is a sign of Perfect Knowledge. Man becomes silent when It is
attained. Then the 'I', which may be likened to the salt doll, melts
in the Ocean of Existence-Knowledge-Bliss Absolute and becomes one
with It. Not the slightest trace of distinction is left.
"As long as his self-analysis is not complete, man argues with much
ado. But he becomes silent when he completes it. When the empty
pitcher has been filled with water, when the water inside the pitcher
becomes one with the water of the lake outside, no more sound is
heard. Sound comes from the pitcher as long as the pitcher is not
filled with water.
"People used to say in olden days that no boat returns after having
once entered the black waters' of the ocean.
"All trouble and botheration come to an end when the 'I' dies. You may
indulge in thousands of reasonings, but still the 'I' doesn't
disappear. For people like you and me, it is good to have the feeling,
'I am a lover of God.'
"The Saguna Brahman is meant for the bhaktas. In other words, a bhakta
believes that God has attributes and reveals Himself to men as a
Person, assuming forms. It is He who listens to our prayers. The
prayers that you utter are directed to Him alone. You are bhaktas, not
jnanis or Vedantists. It doesn't matter whether you accept God with
form or not. It is enough to feel that God is a Person who listens to
our prayers, who creates, preserves, and destroys the universe, and
who is endowed with infinite power.
"It is easier to attain God by following the path of devotion."
The Gospel of Sri Ramakrishna, The Master with Brahmo devotees (I), translated by Swami Nikhilananda
Both Nirguna and Saguna Brahman are experiences of Yogis. A Yogi following the path of devotion experiences Brahman as a person through a layer of Sattva Maya. Hence he interprets his experience as Saguna Brahman. A Yogi who follows Jnana Yoga gains knowledge. What he experiences cannot be defined. This experience when the 'sun of knowledge' dawns is called Nirguna Brahman. The actual position is that no one knows what exactly is Brahman. When experienced through purified ego Brahman appears to be Saguna while when experienced beyond the ego appears to be Nirguna.