  Context. Quasi-periodic pulsations (QPPs) typically display periodic and
regular peaks in the light curves during the flare emissions. Sometimes, QPPs
show multiple periods at the same wavelength. However, changing periods in
various channels are rare. Aims. We report QPPs in a solar flare on 2014
October 27. They showed a period change that depended on whether thermal or
nonthermal components were included. The flare was simultaneously observed by
many instruments. Methods. Using the fast Fourier transform (FFT), we
decomposed the light curves at multiple wavelengths into slowly varying and
rapidly varying signals. Then we identified the QPPs as the regular and
periodic peaks from the rapidly varying signals. The periods are derived with
the wavelet method and confirmed based on the FFT spectra of the rapidly
varying signals. Results. We find a period of 50 s from the thermal emissions
during the impulsive phase of the flare, that is, in the soft X-ray bands. At
the same time, a period of about 100 s is detected from the nonthermal
emissions, such as hard X-ray and microwave channels. The period ratio is
exactly 2.0, which might be due to the modulations of the magnetic reconnection
rate by the fundamental and harmonic modes of magnetohydrodynamic waves. Our
results further show that the 100 s period is present over a broad wavelength,
such as hard X-rays, extreme-UV/UV, and microwave emissions, indicating the
periodic magnetic reconnection in this flare. Conclusions. To our knowledge,
this is the first report about period changes from thermal to nonthermal
components in a single flare that occur at almost the same time. This new
observational finding could be a challenge to the theory of flare QPPs.
