NVIDIA GeForce GTX 780 Ti GPU-Z also reveals the clock speeds of the GPU. The GK110 GPU is clocked at 902 MHz with GPU Boost up to 954 MHz while the memory is clocked at 1502 MHz or 6 GHz effectively. When compared with GTX 780, the upcoming GTX 780 Ti has tiny bits of improvement in terms of specifications. Below is a table which shows the comparison between GeForce GTX 780, GTX 780 Ti and GTX TITAN.
So its not replacing GTX Titan, but its replacing the GTX 780.