Having recompiled the application with EMDK v6.7 I suggest that you use a BSP with the same runtime on the device.
This usually requires installing the latest LifeGuard update on your device.
Talking about your particular case, I don't know of any issue with our EMDK v6.7 on TC51 (which build number and security patch level are you using?).
Which components of the EMDK are you using (Profile API, Barcode API, etc.) and do you've any logs of the crashes?
Our devices have 01-15-01.7 release, looking at releases page seems quite old. On the other hand, upgrading all the field devices with the latest OS updates will require more testing. We are using only Barcode API.The library is crashing on initialization.
In the Android console, I see a DeadObjectException when the app is launched with the 6.7 version of the SDK.