Belligerent Yearling — ruling 1

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.

Belligerent Yearling — ruling 2

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 : règles, fonctionnement et interactions (MTG)

Belligerent Yearling is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Trample Whenever another Dinosaur you control enters, you may have this creature's base power become equal to that creature's power until end of turn.

How it works : Belligerent Yearling

Original English text — Wizards of the Coast / Scryfall

#1November 10, 2023· Wizards of the Coast
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.
No other card is named in this ruling.
#2November 10, 2023· Wizards of the Coast
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.
No other card is named in this ruling.