If I create an RBF transaction using the Qt GUI but specify my own custom change address, bumpfee doesn't work because it claims it can't find the change address:

    "Transaction does not have a change output"

This is because none of the outputs satisfy

    pWallet->IsChange(...)

I see there's a "TODO" commented in `CWallet::IsChange()` in `src/wallet/wallet.cpp`:

    // TODO: fix handling of 'change' outputs. The assumption is that any
    // payment to a script that is ours, but is not in the address book
    // is change.

I tend to put all my addresses in the address book whether I'm going to use them as change or not. I guess if the wallet was capable of creating its own SegWit change addresses I'd stop using custom change addresses, but currently it always seems to send change to a legacy address if I don't manually specify a change address.
CHECK_NONFATAL(min_input_weight == 165);
            if (weight < min_input_weight) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, weight cannot be less than 165 (41 bytes (size of outpoint + sequence + empty scriptSig) * 4 (witness scaling factor)) + 1 (empty witness)");
            }
            if (weight > MAX_STANDARD_TX_WEIGHT) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, weight cannot be greater than the maximum standard tx weight of %d", MAX_STANDARD_TX_WEIGHT));
            }
            coinControl.SetInputWeight(COutPoint(txid, vout), weight);
        }
    }
    if (tx.vout.size() == 0)
        throw JSONRPCError(RPC_INVALID_PARAMETER, "TX must have at least one output");
    if (change_position != -1 && (change_position < 0 || (unsigned int)change_position > tx.vout.size()))
        throw JSONRPCError(RPC_INVALID_PARAMETER, "changePosition out of bounds");
    for (unsigned int idx = 0; idx < subtractFeeFromOutputs.size(); idx++) {
        int pos = subtractFeeFromOutputs[idx].getInt<int>();
        if (setSubtractFeeFromOutputs.count(pos))
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, duplicated position: %d", pos));
        if (pos < 0)
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, negative position: %d", pos));
        if (pos >= int(tx.vout.size()))
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, position too large: %d", pos));
        setSubtractFeeFromOutputs.insert(pos);
    }
    bilingual_str error;
    if (!FundTransaction(wallet, tx, fee_out, change_position, error, lockUnspents, setSubtractFeeFromOutputs, coinControl)) {
        throw JSONRPCError(RPC_WALLET_ERROR, error.original);
    }
}
static void SetOptionsInputWeights(const UniValue& inputs, UniValue& options)