The Cu(hfac)2 complex with a paramagnetic ligand 2-(1-ethyl-imidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L5Et) was synthesized in the form of two polymorphs: α-[Cu(hfac)2L5Et] (bluish green crystals) and β-[Cu(hfac)2L5Et] (brown crystals). Despite the structural similarity of the two modifications, whose solid phases are formed by polymer chains with cis coordination of the hfac ligands, their magnetic properties proved quite different. For β-[Cu(hfac)2L5Et], the repeated cooling-heating cycles at temperatures of 260-180 K lead to a reversible spin transition β-[Cu(hfac)2L5Et]-HT↔β-[Cu(hfac)2L5Et]-LT, resulting from a considerable change in the Cu-ONO distances in the Cu-O•-N< heterospin exchange clusters from 2.312(6) Å (HT form) to 2.011(2) Å (LT form), which leads to a transition of ferromagnetic to antiferromagnetic exchange in them. For α-[Cu(hfac)2L5Et], the first cooling also leads to a decrease in μeff from 2.6 μB at 200 K to 0.35 μB at 100 K on the curve of the μeff(T) dependence. However, this is not a spin transition in the starting α-[Cu(hfac)2L5Et], but, rather, the consequence of the irreversible phase transformation of α-[Cu(hfac)2L5Et] into β-[Cu(hfac)2L5Et] induced by the cooling of the metastable α modification to T < 200 K. When the cooling-heating cycles are repeated further, the reversible spin transition inherent in β-[Cu(hfac)2L5Et] is reproduced. For this reason, all the μeff values recorded in the range of 200-100 K for the cooled α-[Cu(hfac)2L5Et] sample are higher than those for β-[Cu(hfac)2L5Et] up to T ∼75 K until the whole starting α modification transforms into the β form. This effect has never been observed earlier for transition metal complexes with nitroxides. The experimental μeff(T) dependence observed during the first cooling of α-[Cu(hfac)2L5Et] was called a spin transition in the nascent phase to distinguish it from the μeff(T) curve for β-[Cu(hfac)2L5Et] corresponding to the typical reversible spin transition for heterospin complexes of transition metals with organic radicals. The results of thermomagnetic measurements correlate with FTIR and EPR data.