No, Bhagwan Bhairava is not the prior form of Bhagwan Shiva.
The word pūrṇarūpo means complete form not prior form. And here it is said that Lord Bhairava is the complete raudra form of Lord Shankara (viz NilaLohita/Rudra not Lord Shiva).
Shiva Maha Purana 3.8.:
44. O sage, on hearing the haughty words of the lotus-born Brahmā, Śiva became angry as if out for destruction.
- Angrily, he created a Puruṣa Bhairava blazing in great brilliance. Then he spoke lovingly.
4-6. O Kālabhairava, at the outset this lotus-born Brahmā shall be chastised by you. You shine like god of death, hence you are Kālarāja.
You are called Bhairava because you are of terrifying features and you are capable of supporting the universe. Since even Kāla is afraid of you, you are called Kālabhairava.
When you arc angry you will be suppressing the wicked souls. Hence you will be known everywhere as the suppressor of the wicked.
Since you will be devouring the sins of devotees in a trice your name will be famous as sin-eater.
O Kālarāja, you will have forever the suzerainty over my city Kāśī, the city of liberation, which is greater than all other cities.
You alone will be the chastiser of those who commit sins there. Citragupta will write down their auspicious and inauspicious activities.
I hope this clarifies your queries.