How can we demodulate the data in OFDM with different FFT size in OFDM system.

Suppose we are using QPSK demodulation, so when using double frequency sampling, we can get double the FFT-points in order to get the same number of subcarriers. So, after the FFT demodulation, in first case we will get 1024-FFT-point and in second case we will get 2048-FFT-points, so how can we demodulate them in order to get the same number of subcarriers ? what will be different in order to get the same data.

For example:

10 MHz LTE can be sampled with 15.36 Msps and modulated/demodulated with a 1024-point FFT for 15 kHz subcarrier spacing and the 600 subcarriers around DC give the 9 MHz of utilized BW. Alternatively, we can use sampling rate 30.72 Msps (which is 2 * 15.36 Msps) and 2048-point FFT (which is 2*1024-point) to mod/demod and get the same 600 subcarriers.

how will that happen? thank you.