It does not work that way! To reconstruct a signal from its samples the samples have to be taken at a frequency which satisfies the Nyquist criterion: The sampling frequency must be greater than twice the highest frequency in the signal.
The problem with sampling at a lower rate is that an effect called "aliasing" will occur which makes the signal look like a real signal but at the wrong frequency. Aliasing cannot be "processed" out later.
You probably have some idea of the maximum possible frequency for your signal. Set your initial sampling frequency to be greater than twice that maximum frequency. Then you can determine the frequency of the signal and reset the sampling frequency to a lower value if appropriate. Be aware that the Nyquist criterion applies to all signals and noise which appears at the input to the A/D converter. Example: Suppose your signals are guaranteed to be less than 100 kHz. You could start with a sampling frequency of 201 kHz. If your measured signal was at 500 Hz, you could lower the sampling rate to just over 1 kHz. However, any noise or interference in the 501 Hz to 100 kHz band will get into the A/D converter and will appear as an aliased signal or noise in the 0-500 Hz band.
Unless you know the higher frequency noise and interference is very low, you need to change the anti-aliasing filter when you change sampling rates.
Lynn
