Chee,
Start from the project file I uploaded previously. In your attached file the bias point is the original incorrect value, the samples per bit is also too low. If you get the error at the OFDM Demodolation component set the sequence length to a longer value. Try the attached.
Alistu,
You are correct about the bit rate updating automatically, but I imagine you forgot to change the Global Symbol rate to the new value of 2.5 Gbps. This has an effect on the filters and the OFDM Demodulation.