That’s a really good question. I assume that Yugo was not captured and you plan to move the tank through Bulgaria after you activate it.
The OOB rules page 11.
Friendly neutrals may not be attacked, and air units
may not fly over them. They can be moved into (but not
through) as a noncombat move by land units of a power
that is at war (see “Noncombat Move,†page 22). This
moves the territory out of its neutral status, however. The
first friendly power to do so places its national control
marker on the former friendly neutral territory, and its
national production level is adjusted upward by the value
of the territory. With the territory’s loss of neutrality in
this way, its standing army is immediately activated.
The units placed belong to the power that now controls
the formerly neutral territory, and may be used freely
beginning on that power’s next turn.
You would think it to be possible but the rules state that They can be moved into (but not through) as a noncombat move by land units of a power that is at war. I would think that rule would trigger at the beginning of the EOC phase and hold true until the end of that phase regardless of the change in ownership of other territories. So I would say you could not move the tank through that territory.