Yes, and this should be done conscientiously because choosing a different primary string can change which variant strings are allocatable or blocked. The LGR Tool can be used with the RZ-LGR to help applicants find out if there are any variant strings, and if so, which variant strings are allocatable or blocked, and explore options before applying.
For registry operators applying for variant strings of their existing gTLDs, their existing gTLDs are automatically designated as the primary strings and cannot be changed.