Learn Hardcore German: Unser Plan ist heute anders.

ist heute anders: describes the current situation now.
wird heute anders sein: prediction or schedule for later today/future.
hat sich heute geändert: focuses on the change event that occurred today.

Nominative, because it’s the subject of the copular verb ist. With sein, the subject is nominative: "Der Plan ist anders." With a possessive determiner, it remains nominative: Unser Plan ist anders.

Why is it "Unser Plan" and not "unsere/unsers/unseren Plan"?

Because Plan is masculine singular nominative. Possessive determiners (mein, dein, unser, etc.) behave like ein-words: masculine nominative has no ending, hence unser Plan. Quick guide:

feminine nominative/plural → unsere
neuter nominative/accusative → unser
masculine accusative → unseren
dative masc/neut → unserem
genitive masc/neut → unseres
dative fem/gen fem/pl → unserer

What is "anders" here—an adjective or an adverb?

It functions as a predicative adjective (a predicate complement). After sein, adjectives appear in base form (no ending): Der Plan ist anders. It describes the subject’s state, not a noun directly, so no ending is added.

How is "anders" different from "andere/anderes" and from "anders als"?