Hi MSL
What you are trying to do is not uncommon and you have run into an issue that we always have a problem with and that is 2 year ends in the same year.
Unfortunately Dynamics GP does not allow you to name one 2018A and 2018B.
What I have done in the past is to rename the years in the past so that going forward the fiscal year makes sense.
For example you had your previous years with an June 30th year end.
fiscal 2016 would be ending June 30 2016, 2017 was June 30 2017 2018 was June 30 2018.
Now in 2018 you want to switch to a calendar year end. What I had done previously is to do this.
Fiscal 2016 July 1 2015 to June 30 2016 would be come FY 2015 July 1 2015 to June 30 2016
2017 would become FY 2016 July 1 2016 to June 30 2017
2018 would become FY 2017 July 1 2017 to June 30 2018
You would setup a new year called 2018 and it would be a short year July 1 2018 to December 31, 2018
Starting in 2019 the years would be correct so that FY 2019 would become January 1 2019 to December 31 , 2019
This link has instructions on how to do this. It is from an older version of Dynamics GP but the principal is still the same
support.microsoft.com/.../how-to-change-the-fiscal-period-setup-in-general-ledger-in-microsoft-d
The PSTL also has a utility that can assist you with this. I prefer going into SQL and then rebuilding the calendar in the regular Fiscal Calendar setup window
The most important things to remember are:
a) try it out in a test company first
b) make a good backup of your database
c) don't forget to run the Reconcile process
If you are using Fixed Assets there are some additional challenges as well.