Hi Aldevinas,
After consulting with my colleague I got the following info for you:
There is a code in the transaction so that the transaction knows which fiscal year it belongs to. There would be a problem if the 2020 fiscal year was Jan-Dec and the 2021 was also Jan-Dec, and they changed 2020 to be 15 months after some 2021 transactions were entered, but even that change is quite difficult to do as the program doesn't allow it, precisely because it is a sign that there is a big mess ahead.
What you could try to do is a Register Export and change the transactions manually in a spreadsheet, depends on how many there are, of course. You would need to change the fiscal year code as well as the number.
Please let us know if this helps.
Kind regards,
Anna