CI 2.1.0 to CI 3.0.x upgrade - Replacing "encrypt" with the "encryption" library

Yes, it is my worry that we will have issues with our existing data due to potentially differing algorithms.

I am not sure I get what you say here or how to implement it interchanging between the two libraries:

In the worst case, you would need a small script to decode() data using Encrypt and then encrypt() it using Encryption.