What transition occurs when a massive planet accretes too much hydrogen and helium?

It becomes a gas dwarf or a mini-Neptune.

There exists a critical mass threshold for objects that begin forming from rocky and metallic materials. If a planetary embryo grows massive enough, its increased gravity allows it to rapidly pull in and retain vast quantities of light elements, specifically hydrogen and helium, from the surrounding protoplanetary disk while it is still forming. This accretion of lighter material fundamentally changes the planet's chemical makeup, transforming it from a dense, terrestrial object into a less dense world classified as a gas dwarf or a mini-Neptune. This mass dependency acts as a secondary filter determining whether a rocky core retains its terrestrial classification or evolves into a gas-dominated giant.