The highest uniquely resolvable frequency in a sampled signal is the Nyquist frequency, so 75 Hz for your Fs=150 Hz signal. If your highest-frequency signal in your Fs=1500 data are less than 75 Hz, you can use your Fs=150 Hz data. Otherwise, there is no salvaging your Fs=150 Hz data. You cannot recover any frequencies higher than the Nyquist frequency.
As for calculating the frequencies in your signal, use the fft (link) function.
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