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.
{
        return members.count({method, param_idx}) > 0 ? Parse(arg_value) : arg_value;
    }
    
    UniValue ArgToUniValue(std::string_view arg_value, const std::string& method, const std::string& param_name)
    {
        return membersByName.count({method, param_name}) > 0 ? Parse(arg_value) : arg_value;
    }
};
CRPCConvertTable::CRPCConvertTable()
{
    for (const auto& cp : vRPCConvertParams) {
        members.emplace(cp.methodName, cp.paramIdx);
        membersByName.emplace(cp.methodName, cp.paramName);
    }
}
static CRPCConvertTable rpcCvtTable;
UniValue RPCConvertValues(const std::string &strMethod, const std::vector<std::string> &strParams)
{
    UniValue params(UniValue::VARR);
    for (unsigned int idx = 0; idx < strParams.size(); idx++) {
        std::string_view value{strParams[idx]};
        params.push_back(rpcCvtTable.ArgToUniValue(value, strMethod, idx));
    }
    return params;
}
UniValue RPCConvertNamedValues(const std::string &strMethod, const std::vector<std::string> &strParams)