He (Krsna) already exists in multiforms, but none of the forms are material. Sridhara Svami has commented that arupinah, "without form," means without material form
SB Canto 3
My dear Lord, although You have no material form, You have Your own innumerable forms. They truly are Your transcendental forms, which are pleasing to Your devotees.
In the Brahma-saṁhitā it is stated that the Lord is one Absolute, but He has ananta, or innumerable, forms. Advaitam acyutam anādim ananta-rūpam (Bs. 5.33). The Lord is the original form, but still He has multiforms. Those multiforms are manifested by Him transcendentally, according to the tastes of His multidevotees. It is understood that once Hanumān, the great devotee of Lord Rāmacandra, said that he knew that Nārāyaṇa, the husband of Lakṣmī, and Rāma, the husband of Sītā, are one and the same, and that there is no difference between Lakṣmī and Sītā, but as for himself, he liked the form of Lord Rāma. In a similar way, some devotees worship the original form of Kṛṣṇa. When we say "Kṛṣṇa" we refer to all forms of the Lord—not only Kṛṣṇa, but Rāma, Nṛsiṁha, Varāha, Nārāyaṇa, etc. The varieties of transcendental forms exist simultaneously. That is also stated in the Brahma-saṁhitā: rāmādi-mūrtiṣu. .. nānāvatāram. He already exists in multiforms, but none of the forms are material. Śrīdhara Svāmī has commented that arūpiṇaḥ, "without form," means without material form. The Lord has form, otherwise how can it be stated here, tāny eva te 'bhirūpāṇi rūpāṇi bhagavaṁs tava: "You have Your forms, but they are not material. Materially You have no form, but spiritually, transcendentally, You have multiforms"? Māyāvādī philosophers cannot understand these transcendental forms of the Lord, and being disappointed, they say that the Supreme Lord is impersonal. But that is not a fact; whenever there is form there is a person. Many times in many Vedic literatures the Lord is described as puruṣa, which means "the original form, the original enjoyer." The conclusion is that the Lord has no material form, and yet, according to the liking of different grades of devotees, He simultaneously exists in multiforms, such as Rāma, Nṛsiṁha, Varāha, Nārāyaṇa and Mukunda. There are many thousands and thousands of forms, but they are all viṣṇu-tattva, Kṛṣṇa.