Simply allows you to have long or short fiscal 'years' as needed, but the end date of your current fiscal period CANNOT BE EARLIER THAN THE LATEST TRANSACTION YOU HAVE RECORDED IN THAT PERIOD.

For example, if your fiscal period is currently set as January 1 2018 to December 31 2018, and your latest transaction recorded in August 7 2018, you could change the fiscal period to be January 1 to August 7 2018, but not January 1 to August 6 2018.

So you cannot go back and chop an existing fiscal 'year' that already had transactions into pieces.

IF you have to fix your problem within Simply, you will need to go back and re-do a whole pile of bookkeeping.  However, before you do this, I suggest checking with your accountant.  I work in an accounting office, and one of the jobs I do quite frequently is manually 'chop' Simply data up into shorter fiscal years because of amalgamations, etc.  Here's how it works:

1.  The bookkeeper sets the Simply file so that the 'year' ends at the next correct yearend date - if I am reading your post correctly, you wanted to have years as follows:  (a) March 1 2016 - Feb 28 2017, (b) March 1 - March 15 2017, and (c) March 16 2017 - Feb 28 2018.  As the bookkeeping is done up to Nov'17, the earliest correct yearend date you can set is February 28, 2018, so your Simply file will have one year (March 1 '17 - Feb 28 '18) that combines two of the accountant's years.

2.  The bookkeeper gives the Simply file to the accountant as usual.  If the accountant is just planning to do the March 15'17 yearend, then this could be done as soon as the March 1-15 bookkeeping is finished.

3.  Instead of pulling all of the data from the Simply file into their yearend file, the accountant only pulls in the March 1-15 transactions. 

4.  Once the bookkeeping is done for to Feb 28'18, the bookkeeper gives the Simply file to the accountant again.  At this point, someone in the accountant's office has to strip out the Mar 1-15 transactions, and do a little extra work to manually close the March 1-15 income and expenses to retained earnings.  If I were doing it, I would dump the March 1'17 - Feb 28'18 transactions into Excel, do a bit of data manipulation, and then pull the Excel data into our yearend software.  It rarely takes me more than an hour, which is much more efficient than re-doing months of bookkeeping.