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
Thank you! Worked wonderfully… I’d been through ADSI edit and cleared any reference to the failed 5.5 server for the site I’m working on, but as this had no reference to the old server I didn’t touch it at the time.