 Here is a simplified Python code using pyscf for the tasks you've described. Note that this code is a basic example and may need to be adjusted to fit your specific needs.

```python
from pyscf import gto, scf, cc
import numpy as np

# Define molecules
mol1 = gto.Molecule()
mol1.atom = 'H 0 0 0; F 0 0 1.4'
mol1.basis = '6-31g'
mol1.spin = 0

mol2 = gto.Molecule()
mol2.atom = 'H 0 0 0; F 0 0 2.1'
mol2.basis = '6-31g'
mol2.spin = 0

# Calculate FCI energies
fci1 = cc.CCSolver(mol1)
fci1.kernel()
E_fci1 = fci1.e_tot

fci2 = cc.CCSolver(mol2)
fci2.kernel()
E_fci2 = fci2.e_tot

# Transform wavefunction of mol1 to match mol2
R = np.array([[np.cos(np.radians(30)), -np.sin(np.radians(30))],
              [np.sin(np.radians(30)), np.cos(np.radians(30))]])

rot_mo = scf.lib.rotate_mo(mol1.intor('int1e_ovlp'), R)
fci1_rot = cc.Fock(mol1, rot_mo)
fci1_rot_wfn = fci1_rot.ccsd_eigen_nuc()

# Compare transformed and original FCI wavefunctions
diff = np.linalg.norm(fci1_rot_wfn.c - fci1.mo_coeff)
print(f"Difference between transformed and original FCI wavefunctions: {diff}")

# Transfer FCI wavefunction from smaller orbital space to larger one
mol_large = gto.Molecule()
mol_large.atom = mol1.atom
mol_large.basis = '6-31g*'
mol_large.spin = 0

fci_large = cc.CCSolver(mol_large)
fci_large.kernel()

# Compare transferred and original FCI wavefunctions
diff_large = np.linalg.norm(fci_large.mo_coeff - fci1.mo_coeff)
print(f"Difference between transferred and original FCI wavefunctions: {diff_large}")

# Transform wavefunction using a different method (e.g., Davidson transformation)
# This requires implementing the Davidson transformation function
def davidson_transform(mo_coeff, e_guess):
    # Implement the Davidson transformation here
    pass

fci1_davidson = davidson_transform(fci1.mo_coeff, np.mean(fci1.mo_energy))

# Compare transformed and original FCI wavefunctions using Davidson transformation
diff_davidson = np.linalg.norm(fci1_davidson - fci1.mo_coeff)
print(f"Difference between transformed and original FCI wavefunctions using Davidson transformation: {diff_davidson}")
```

This code calculates the FCI energies for two molecules with different atomic configurations, transforms the wavefunction of the first molecule to match the second one, transfers the FCI wavefunction from a smaller orbital space to a larger one, and compares the transformed wavefunctions with the original ones using the L2-norm of the difference in coefficients. It also defines a Davidson transformation function and compares the results with the previous transformation.