  Oscillations in the solar atmosphere have long been observed in quiet
conditions, and increasingly also in data taken during solar flares. The
chromosphere is known for its 3-minute signals, which are particularly strong
over sunspot umbrae. These signals are thought to be driven by photospheric
disturbances and their periods determined by the chromosphere's acoustic
cut-off frequency. A small number of observations have shown the chromospheric
3-minute signals to be affected by energetic events such as solar flares,
however the link between flare activity and these oscillatory signals remains
unclear. In this work we present evidence of changes to the oscillatory
structure of the chromosphere over a sunspot which occurs during the impulsive
phase of an M1 flare. Using imaging data from the CRISP instrument across the
H$\alpha$ and Ca II 8542 {\AA} spectral lines, we employed a method of fitting
models to power spectra to produce maps of areas where there is evidence of
oscillatory signals above a red noise background. Comparing results taken
before and after the impulsive phase of the flare, we found that the
oscillatory signals taken after the start of the flare differ in two ways: the
locations of oscillatory signals had changed and the typical periods of the
oscillations had tended to increase (in some cases increasing from $\lt$100s to
$\sim$200s). Both of these results can be explained by a restructuring of the
magnetic field in the chromosphere during the flare activity, which is backed
up by images of coronal loops showing clear changes to magnetic connectivity.
These results represent one of the many ways that active regions can be
affected by solar flare events.
