If a user walks through the New Company wizard setup, they are asked for the start date for the current fiscal year.  This interferes with prior year Data Migration (prior year Trial Balances) and the only solution today is to discard the entire company setup and recreate a new one, this time answering the "Start Date" with the prior year (or earlier).

Accounting Periods in D365F do not allow creation of prior-year periods after the first set is created and "Date Locked" = yes.  This is true even if there are no General Ledger Entries.  In Dynamics NAV, if this situation comes up the partner can directly 'clear' the Accounting Periods and recreate them.  In D365F the partner does not have access to rectify the situation and the only solution is recreating the company from scratch (copy company will also recreate the same problem in Accounting Periods).  It's not an acceptable scenario to configure a company only later to discover a problem with prior year data migrations.