Copy the 2017 file and rename it XXX-ORIG17. Now massage all the data to delete the 2017 data and update the beginning balance sheet and accum depreciation. It might be easier to proforma 2016 again and start with that file, then you don't have new input to delete. Just correct beg balance sheet and accum depreciation to match the 2017. Also the new proforma won't have any e-file actitivity.
Screen 1 - Change FYE to June 30. Screen 4 - adjust input for short year. Do your part year input, possibly putting a date of sale on the assets to get the depreciation right and review carefully. You should now have a 2017 Form that has on page 1 "For Calendar year 2017 or tax year beginning 1/1/2018 and ending 6/30/2018
PS: But great answer George, very detailed. If I ever have to do a short-year final return in Lacerte before the forms are released, I'm calling you!