chore(common): update UPGRADING notes

UPGRADING.md

@@ -1,5 +1,67 @@
 # Upgrading Guide
 
+## From `Concrete v1` To `Concrete v2`
+
+### Encrypt/Run/Decrypt type signatures are changed
+
+In `Concrete v1`, there were `PublicArguments` and `PublicResult` types. `PublicArguments` was used to encapsulate encrypted and clear arguments to the function and `PublicResult` was used to store the encrypted result of the function.
+
+```python
+x, y, z = 10, 20, 30
+encrypted_args = circuit.encrypt(x, y, z)
+encrypted_result = circuit.run(encrypted_args)
+result = circuit.decrypt(encrypted_result)
+```
+
+where `encrypted_args` is of type `fhe.PublicArguments`, and `encrypted_result` is of type `fhe.PublicResult`.
+
+This was simple, but limiting (e.g., since `PublicArguments` contained all arguments, a change in one of the arguments required re-encryption of all arguments).
+
+In `Concrete v2`, there is `fhe.Value` type, which is the result of all of encrypt, run, and decrypt.
+
+```python
+x, y, z = 10, 20, 30
+encrypted_x, encrypted_y, encrypted_z = circuit.encrypt(x, y, z)
+encrypted_result = circuit.run(encrypted_x, encrypted_y, encrypted_z)
+result = circuit.decrypt(encrypted_result)
+```
+
+where all of `encrypted_x`, `encrypted_y`, `encrypted_z` and `encrypted_result` are `fhe.Value`.
+
+In case `y` value needs to be changed whereas the others need to remain, it's possible to do:
+
+```python
+new_y = 42
+_, new_encrypted_y, _ = circuit.encrypt(None, new_y, None)
+
+new_encrypted_result = circuit.run(encrypted_x, new_encrypted_y, encrypted_z)
+new_result = circuit.decrypt(new_encrypted_result)
+```
+
+New `fhe.Circuit.run(...)` also support tuples so the old code still works! However, `fhe.Server.run(...)` had another parameter called `evaluation_keys`, which have to be a keyword argument now as function arguments are variadic now!
+
+```python
+server: fhe.Server = ...
+encrypted_x: fhe.Value = ...
+encrypted_y: fhe.Value = ...
+encrypted_z: fhe.Value = ...
+evaluation_keys: fhe.EvaluationKeys = ...
+
+server.run(encrypted_x, encrypted_y, encrypted_z, evaluation_keys=evaluation_keys)
+                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+                                                  this needs to be a keyword argument now
+```
+
+Whereas previously it could be:
+
+```python
+server: fhe.Server = ...
+encrypted_args: fhe.PublicArguments = ...
+evaluation_keys: fhe.EvaluationKeys = ...
+
+server.run(encrypted_args, evaluation_keys)
+```
+
 ## From `Concrete Numpy v0.x` To `Concrete v1`
 
 ### The PyPI package `concrete-numpy` is now called `concrete-python`.