changing frequency of a time domain signal by either frequency multiplication or frequency division

I have a periodical signal with period of 1 sec (and samling rate Fs=1000 HZ). Now, I'd like to use FFT and iFFT to change the period of the signal such that the period could be 1.2 sec or 0.8 second.

[Star Strider]

The easiest way to do this is with the resample function. The interpft function already does what you want to do.

[Chunru]

How about just change sampling frequency from 1000 to 1000/1.2 or 1000/0.8 so that the original signal of 1sec period becomes 1.2 sec and 0.8sec?