According to valence bond theory, atomic orbitals of slightly different mix and recast to form new orbitals of identical energies and properties. This process as called hybridization and, new generated orbitals are referred as hybridized orbitals.
Also, according to assumption of VBT, only central atom undergoes hybridization.
In case of HNO3, N is a central atom and it undergoes sp2 hybridization in HNO3. The structure of HNO3 is planner.
Further, HNO3 comprises of H+ and NO3- ion. Here, role of H+ is to act as counter ion and balance the charge on NO3-
Now, when HNO3 is mixed in water, it undergoes dissociation to generate H+ and NO3- ion. Here, parent ion i.e. NO3- is not disturbed. Hence, it's hybridization will not change. Only there will be minor variation in bond length and bond angle because of interaction of NO3- with H2O molecules.