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.
namespace feebumper {
enum class Result
{
    OK,
    INVALID_ADDRESS_OR_KEY,
    INVALID_REQUEST,
    INVALID_PARAMETER,
    WALLET_ERROR,
    MISC_ERROR,
};
bool TransactionCanBeBumped(const CWallet& wallet, const uint256& txid);
/** Create bumpfee transaction based on feerate estimates.
 *
 * @param[in] wallet The wallet to use for this bumping
 * @param[in] txid The txid of the transaction to bump
 * @param[in] coin_control A CCoinControl object which provides feerates and other information used for coin selection
 * @param[out] errors Errors
 * @param[out] old_fee The fee the original transaction pays
 * @param[out] new_fee the fee that the bump transaction pays
 * @param[out] mtx The bump transaction itself
 * @param[in] require_mine Whether the original transaction must consist of inputs that can be spent by the wallet
 */
Result CreateRateBumpTransaction(CWallet& wallet,
    const uint256& txid,
    const CCoinControl& coin_control,
    std::vector<bilingual_str>& errors,
    CAmount& old_fee,
    CAmount& new_fee,
    CMutableTransaction& mtx,
    bool require_mine,