Is it possible to change an output of a partially signed transaction? In step 5 of the procedure described in https://en.bitcoin.it/wiki/Smart_Property#Theory, the buyer creates a partially signed transaction. Will the seller be able to change an output prior to signing and broadcasting - and thus returning the token to an output he may redeem himself?
In one word: no. Changing tx output after signing makes existing signature invalid.

( There are some special cases, like signature hash type SIGHASH_SINGLE or SIGHASH_NONE, but we assume that the buyer uses correct signing SIGHASH_ALL )