Hi Nicola,
please look at the third party plug-ins running in your instance too: some of them could have some issues.
For the record:
- https://docs.moodle.org/28/en/New_features#Gradebook_improvements
- https://docs.moodle.org/dev/Moodle_2.8_release_notes#Gradebook
If you want to go for the safest path - again, third party plug-ins reqs should be checked even before starting this path - you should clone your current instance in the new server (given the server compatibility with both 2.3.1, https://docs.moodle.org/dev/Moodle_2.3_release_notes#Requirements, and 3.3, https://docs.moodle.org/dev/Moodle_3.3_release_notes#Server_requirements) and update the cloned instance up to 3.3.
Warm suggestion: whatever the path will be, please test the migration process in a separate sandbox .
HTH,
Matteo