It is best practice not to mix different lubricating greases. Since the grease in the bearing is unknown, the bearing should be purged with the replacement product until >90% of the old product has been displaced. In addition, the bearing should be monitored for any signs of grease incompatibility (grease softening excessively, bearing running hotter than normal, etc.). It may be necessary to shorten the relubrication interval to complete the displacement of the old grease from the bearing. Update maintenance records as appropriate.