We derive metrics for adaptation of antenna-element spacing in nonstationary pure line-of-sight MIMO (LOS-MIMO) links. An example of such a deployment is a high-frequency and very high-speed wireless link between an indoor base-station and an untethered mixed-reality headset worn by a user whose distance from the base-station is time-variant. Due to the narrow antenna beams required to close the link budget and highly directional propagation at high frequencies, achieving high transmission spectral efficiency must rely on LOS-MIMO transmission—realization of which depends critically on the distance between transmitter and receiver, as well as on the antenna element spacing at the transmitter and receiver. Therefore, metrics that facilitate quick adaptation of the antenna element spacings in response to changes in the LOS-MIMO link length are required. We derive these metrics for MIMO orders two, three and four (i.e., number of transceiver antennas) and show how the transmission rate of the LOS-MIMO link depends on them. We also show how they can be quickly computed from simple measurements of path phase.