I just found some old notes and I tested this process. 

Note the S Corp client number – let’s say S005 FOR TAX YEAR 2021 where last digit of file is a 1.

Go to the partnership module and create a new client S005 and enter Screen 1 info.

Close Lacerte and open File Explorer and go to your data drive shown by X. X:>Lacerte>21tax>SDATA>DETAIL and locate S005.SD1

Copy that file

go to X:>Lacere>21tax>PDATA>DETAIL and paste that file S005.SD1

Rename P005.PD1 to P005.old

Rename S005.SD1 to S005.PD1

Now return to Lacerte and MOST OF YOUR DATA will now be in Partnership module. Review carefully. Not a perfect solution, but I think quicker than total re-entering of all data.

This same concept will work to go Partnership to S Corp