Deploy CDT with out disturbing existing instances

There will Problem with the SubProcess.
1. New CDT is treated as latest version. So the process model which use the cdt will pick the latest version when that process instance starts. Sub Process will always pick the latest version of the CDT.
2. suppose there are some running process instances that are using the old CDT Version then the Process Variables will display with
^[Version of the CDT] symbol eg: AMS_AnimalData^7
3. Suppose you have a Parent Process model let the name is A and you are using the older version of CDT. This process model contains a subprocess B a and you are passing that CDT value to B doing some modification to the CDT with the including the newly added fields then getting back to A. Now For some reasons the are many nodes(may be user Input tasks) between A and B.
In this case the process instance of A is running in other environment with old CDT Version and now the process flow hasn't reached to subprocess B. If you deploy your latest version of your CDT, along with the change in the Subprocess B using the newly added CDT fields, then the subprocess B will pick your latest version.
Now the changes that are updated to the rest of the three fields will not get back to Parent Process A.and this may result unExpected behaviour in Parent Process A.

So if you add only the CDT fields and you haven't changed anything in the Process Models that are using that CDT. I think there will not be any impact for your instances.

There will Problem with the SubProcess.
1. New CDT is treated as latest version. So the process model which use the cdt will pick the latest version when that process instance starts. Sub Process will always pick the latest version of the CDT.
2. suppose there are some running process instances that are using the old CDT Version then the Process Variables will display with
^[Version of the CDT] symbol eg: AMS_AnimalData^7
3. Suppose you have a Parent Process model let the name is A and you are using the older version of CDT. This process model contains a subprocess B a and you are passing that CDT value to B doing some modification to the CDT with the including the newly added fields then getting back to A. Now For some reasons the are many nodes(may be user Input tasks) between A and B.
In this case the process instance of A is running in other environment with old CDT Version and now the process flow hasn't reached to subprocess B. If you deploy your latest version of your CDT, along with the change in the Subprocess B using the newly added CDT fields, then the subprocess B will pick your latest version.
Now the changes that are updated to the rest of the three fields will not get back to Parent Process A.and this may result unExpected behaviour in Parent Process A.

So if you add only the CDT fields and you haven't changed anything in the Process Models that are using that CDT. I think there will not be any impact for your instances.