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.
[&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
        {
            std::shared_ptr<CWallet> const pwallet{GetWalletForJSONRPCRequest(request)};
            if (!pwallet) return UniValue::VNULL;
            
            
            pwallet->BlockUntilSyncedToCurrentChain();
            UniValue options{request.params[4].isNull() ? UniValue::VOBJ : request.params[4]};
            InterpretFeeEstimationInstructions(request.params[1], request.params[2], request.params[3], options);
            PreventOutdatedOptions(options);
            std::set<std::string> addresses_without_amount;
            UniValue recipient_key_value_pairs(UniValue::VARR);
            const UniValue& recipients{request.params[0]};
            for (unsigned int i = 0; i < recipients.size(); ++i) {
                const UniValue& recipient{recipients[i]};
                if (recipient.isStr()) {
                    UniValue rkvp(UniValue::VOBJ);
                    rkvp.pushKV(recipient.get_str(), 0);
                    recipient_key_value_pairs.push_back(rkvp);
                    addresses_without_amount.insert(recipient.get_str());
                } else {
                    recipient_key_value_pairs.push_back(recipient);
                }
            }
            if (addresses_without_amount.size() == 0) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Must provide at least one address without a specified amount");
            }
            CCoinControl coin_control;
            SetFeeEstimateMode(*pwallet, coin_control, options["conf_target"], options["estimate_mode"], options["fee_rate"], false);
            coin_control.fAllowWatchOnly = ParseIncludeWatchonly(options["include_watching"], *pwallet);