If a continuous effect gives a creature a new card type, does that override its original types?

Continuous effects that add a card type use the 'layer' system defined in the Comprehensive Rules, specifically Layer 4 (type-changing effects) under CR 613.1d. By default, granting a new type is additive — it stacks on top of the existing types rather than replacing them.

CR 205.1b clarifies that an object can have multiple card types simultaneously (e.g., an Artifact Creature, or an Enchantment Land). So if an effect says 'this creature becomes an artifact,' it is now both a creature and an artifact, retaining all of its previous types and subtypes.

The only time original types are overridden is when the effect explicitly includes language such as 'in addition to its other types' (which is just clarifying it keeps types) or — more relevantly — when an effect uses language like 'becomes a [type]' without 'in addition to its other types,' which some older cards use. Even then, modern templating and CR 205.1b interpretation means most effects are additive unless they say 'it loses all other types' or similar.

Example: Your Grizzly Bears (a Creature) is targeted by an effect that makes it an artifact until end of turn. It is now an Artifact Creature — it retains the Creature card type and gains Artifact. It is still a Bear, still a creature for combat purposes, and is now also affected by artifact-matters cards like Shatter.

If instead an effect read 'it becomes an artifact and loses all other card types,' only then would it stop being a creature. Always read the effect carefully for explicit removal language (CR 613.1d, 205.1b).