Dear Grace,
please read my blog for this topic:
I assume, that you use currently the SAP delivered fiscal year variant 10. In this case you have no possibility to introduce a shortened fiscal year and to adjust your fiscal year variant afterwards. The only possibility we can recommend in such case ist to set up a new company and migrate the data to the new company with the set of books with the new fiscal year variant.
Maybe at this point of reimplementation it is a good approach to use "other fiscal year" instead of fiscal year variants based on calendar year, because here some changes might be possible.
Best regards
Marlene