It's impossible to perform the operating system replacement (Windows CE from/to Windows Mobile or Windows CE from/to Android) without sending the unit to the Zebra Repair Center. Some device types require hardware changes.
Subversions OS upgrade is the matter of OEM OS Update and for customers with valid service contracts the upgrade is free. In this terms you can upgrade Windows Mobile 6.1 to Windows Mobile 6.5 on MC3190, MC55 and MC75. The similar rules apply to Android OS.
Each OS change means setting to factory defaults. All user applications and settings are removed.