Codesys v2.3 only has OPC-DA (Availability depends on PLC model and manufacturer. Even though OPC-UA may not always be available on a PLC with Codesys v3.5, check your vendor's documentation to check if is a optional).
There are Gateways (hardware and software) that allow "conversion" between OPC-DA and OPC-UA or between other protocols for OPC-UA, for example, if your PLC has Modbus-TCP/RTU. It is also possible to use another PLC, as Camile G. commented, but possibly the cost and development should be more complicated, depending on the case it may be worth migrating the entire system to a single PLC with Codesys v3.5 and OPC-UA.