Possible to change receive address in signed txn?

Wondering if there is a way to change the receiving address so when calling future_msgpack_decode on a signed transaction the receiver being returned is not the address the transaction is actually being sent to?

It depends.

For normal signature or multisig signatures, changing any field in the transaction would invalidate the signature. The altered transaction would be rejected systematically by the blockchain. There is no way to modify the signature to make the transaction valid (unless some catastrophic cryptographic break happen which would most likely break a huge part of the Internet even).

For logicsig/smart signatures, it depends on the smart signature program. In the extreme case of the int 1 program, you can change the transaction as you see fit.

Thanks for the reply.
So for the case of logicsig/smart sig could the transaction be signed from a smart contract with the extreme case of int 1? Could you go more into detail about that?

I would recommend reading about smart signatures there: Introduction - Algorand Developer Portal

The int 1 example is extreme: it has no use in a real code.
It can be used sometimes when playing with Algorand features.