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.
{"estimate_mode", UniValueType(UniValue::VSTR)},
                {"outputs", UniValueType()}, 
            },
            true, true);
        if (options.exists("confTarget") && options.exists("conf_target")) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "confTarget and conf_target options should not both be set. Use conf_target (confTarget is deprecated).");
        }
        auto conf_target = options.exists("confTarget") ? options["confTarget"] : options["conf_target"];
        if (options.exists("replaceable")) {
            coin_control.m_signal_bip125_rbf = options["replaceable"].get_bool();
        }
        SetFeeEstimateMode(*pwallet, coin_control, conf_target, options["estimate_mode"], options["fee_rate"], false);
        
        if (!options["outputs"].isNull()) {
            if (options["outputs"].isArray() && options["outputs"].empty()) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, output argument cannot be an empty array");
            }
            CMutableTransaction tempTx;
            AddOutputs(tempTx, options["outputs"]);
            outputs = tempTx.vout;
        }
    }
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    EnsureWalletIsUnlocked(*pwallet);
    std::vector<bilingual_str> errors;
    CAmount old_fee;
    CAmount new_fee;