Effects that modify the power of Belligerent Yearling without setting it to a specific value will apply to the new base power no matter when they started to take effect. The same is true for counters that change its power.

Belligerent Yearling's ability overwrites any previous effects that set its power, including previous instances of that ability. Other effects that set its power to specific values that start to apply after the ability resolves, including future instances of the ability, will overwrite that effect.