Such a shift is now in fact limited to monosyllabic and polnoglasie stems, while it is excluded in polysyllabic and derived stems, and in a number of one-vowel stems (for examples, see Ch. V, The Adjective, §3.1, p. 203.).
This pattern suggests that the new vowel wa indeed an inserted one, and not a genuine polnoglasie development, in which the new vowel should be fixed (as in, for example, CSR город/gorod-Ø 'town', NS ~ города/gorod-a GS.)