Rebuilding the data mart after changing the fiscal years is the only thing you should need to do. A couple of things you can check:
Check the fiscal periods in the data mart. Run this statement and find the ID for the company in question--
select * from datamart.organization
Then run this to find the fiscal years--
select * from Datamart.FiscalCalendarView where OrganizationId = x and FiscalYear = 2021
The results will show the period start and end dates.
You can also run these two statements as well--
select * from Datamart.FiscalYear where OrganizationId = x
select * from Datamart.Period where OrganizationId = x
I'd also run these two statements against the GP company database and look for any missing years or overlaps. I've seen issues where users will create a short year when switching from a fiscal year to a calendar year. Sometimes they short years get labeled with something that is out of sequence, like calling it 2220. When that happens, MR is expecting all years between 2021 and 2220.
select PERNAME, PERIODDT, PERDENDT, PERIODID, YEAR1 from sy40100 where Series = 0 order by year1, PERIODID
select * from SY40101
