It is a pity that we are discussing this when I have no time to do some research on this, but here comes the solution from what I remember: From the point of view the then Hungary (I assume!), its borders have not changed in 1918, but from the point of view of all other states (CS, RO etc. + the winners of WWI + consequently all other relevant states), (1) the new countries and governments of CS, RO, YUG were recognized (some of them even as early as in the summer of 1918), (2) their temporary borders were set at variou times during 1919 (not later then with the treaty of St. Germain). In addition, the new states were de-facto under the control of the new governments (for Slovakia this happened by early 1919 with a subsequent interruption however). In sum, Hungary's borders changed in late 1918/early 1919 de-facto and de-iure from the point of view of all countries except de-iure from the point of Hungary.  In addition, I consider probable that Hungary was forced by a part of the Treaty of Trianon to accept the changes in retrospect, but that would require some research. Conclusion: It is certainly "more" correct to treat the borders as changed as from 1918 with the corresponding comments of course.