Can microbursts occur at high altitudes?
Strong downbursts of air are possible at high levels also, for example in Cumulonimbus-clouds. However I wouldn't characterize them as microbursts, since the change of airflow direction is an equally dangerous and defining factor in microbursts.

(source: weather.gov.hk)
The head/tailwind parts don't exist at higher altitudes and you would experience the downdraft merely as turbulence.