Problem will be when the 115kbaud side sends 500kbyte worth of data, and it should be send out to the 9600 baud side - this is 100 times slower. You would need to buffer nearly all of the data, and even a PSoC5 hasn't got enough memory for it.
When you can do flow control, you can stop the sender whenever the buffer is about to overflow. I think you could do this in hardware using a DMA (not in all PSoC4 devices) and some other logic. But its easier to do this in software.
The CY8CKIT-044 PSoC 4 M-Series Pioneer Kit has 1Mbit FRAM (=128kB), which can be used to make a circular buffer. If you can ease the specs on the amount of data transferred in one batch from 115kBd to 9.6kBd UART, then it can be accomplished.