in Osprey it says that the "Vladivostok" uniform was introduced in October 1918, and were "widely adopted by summer 1919" but not everywhere - I guess the closer to Vladivostok the more probable.
the more distinctive change would have been the gradual introduction of the left shoulder shield patches from early 1918 on.
I would regard rather this as a "cut", when the need arose to look different from the Bolsheviks. The uniform pieces would not be so important, everyone wore what was available, especially in winter. The markings are important