Well, I also forget to report back in this issue. Sorry.
As expected, they did an upgrade (via softaculous) ant it breaks the system.
I deleted all the tables that were not in a clean OJS 3.1.1-0.
In my case the query to clean the DB I need to
DROP TABLE `books_for_review`, `books_for_review_authors`, `books_for_review_settings`, `captchas`, `categories`, `category_settings`, `comments`, `dataverse_files`, `dataverse_studies`, `edit_assignments`, `email_templates_settings`, `external_feeds`, `external_feed_settings`, `groups`, `group_memberships`, `group_settings`, `mutex`, `objects_for_review`, `object_for_review_assignments`, `object_for_review_persons`, `object_for_review_settings`, `paypal_transactions`, `pln_deposits`, `pln_deposit_objects`, `processes`, `publications`, `publication_categories`, `publication_galleys`, `publication_galley_settings`, `publication_settings`, `referrals`, `referral_settings`, `review_object_metadata`, `review_object_metadata_settings`, `review_object_types`, `review_object_type_settings`, `roles`, `subeditor_submission_group`, `submission_xml_galleys`, `theses`;
Then I go with the progressive upgrade that Antti-Jussi suggested… first baby-steps, version by version (with docker it’s easy 
) and after some try-error I discover my minimal migration path was:
3_1_1-0 > 3_1_2-4 > 3_2_0-3 > 3_2_1-1
After this process I was able to upgrade to the last version but I found some extra errors related with the journal’s about field… and thanks to Nate, I was able to debug and find the offending field:
Finally I got some trouble to login… but was something wired with my browser cache because after cleaning server’s cache, and ensure I was on php 7.3 I changed from Friefox to chromium and all worked like a charm.
So, checking the post as solved.
Thanks you all for your help.