If you find that you are unable to change the Exchange mode from Mixed to Native and have removed all exchange servers the likely cause is because a site replicator still exists. try the following steps to see if this rectifies the issue:
Log on to Your exchange server
Open Exchange System Manager
Expand Tools -> Site Replication Service
Right Click and Delete ‘Site Replication (<SERVERNAME>)’
Open Exchange System Manager
Right Click ‘Organisation’ <ORGNAME> (<SERVERNAME>)
Select Properties
Click Change Mode
Click Yes to Except Native Mode