Change ID of a class

I don't think that's possible, but still, what's the reason they have to be the same? If used in scripts or VAFs, best practices dictate using Aliases instead of IDs

because it is a client that already has everything configured by ID, nothing by alias. It has a rather complex structure always calling the IDs

You may need to create a new development vault as a copy of the production vault. That should ensure that ID's are the same for now. But parity will soon be broken when you start adding new stuff in the development vault. As Ne0Tr0n said, you should go through existing configuration and change IDs to Aliases.

A ok thank you very much, so there is no way to change an ID of a class. The aliases is complex, it is easily a job of more than a month dedicated only to that.

Tough luck but also a consequence of not taking the necessary steps from the beginning, or not correcting the policy soon after aliases became available, if the vault is older than that. The longer you wait the bigger the effort to change.