These will work correctly:
upsert({_id:FOO},

{key:BAR, other:1}

)
upsert({_id:FOO, key:BAR}, {$set: {other: 1}})
upsert({_id:FOO, key:BAR},

{other:1}

) // converted to the first example.

This must fail:
upsert({_id:FOO, key:BAR},

{key:BAZ}

)

This can cause dup IDs but wont fail until/unless we get unique indexes working across shards:
upsert({_id:FOO, key:BAR}, {$set: {other:1}})
upsert({_id:FOO, key:BAZ}, {$set: {other:1}})