If exposed to high enough temperatures and pressures, long-buried igneous rock could

Metamorphic rock

Explanation:

Igneous rocks are the rocks that are formed due to the crystallization of magma at or below the earth's surface. The size of the crystals in these rocks is directly dependent on the rate of cooling. These igneous rocks contain magma with either a high amount of silica or a lesser amount of silica. For example, Granite, Rhyolite, Syenite.

These igneous rocks when subjected to the extremely high pressure and temperature condition, then it turns into a metamorphic rock. These are marked by the higher grade of metamorphism. They are commonly found on the earth at certain depths.  

In general, metamorphic rocks are the secondary rocks that are directly derived from the pre-existing sedimentary, metamorphic or igneous rocks. For example, Schist, Gneiss, Quartzite.

Hence, an igneous rock can change into a metamorphic rock due to the increasing temperature and pressure condition.

it cools slower in the earth meaning the crystals have more time to form than lava on the surface