Yes, you can but there are a few steps in order to complete this.

1. Export all the assignments with grades via the Gradebook Print out found via
  • Gradebook
  • Reports
  • Gradebook Print out

2. Create a Letter Grade Scale as per Letter Grade Scale

3. Remove all assignments from the Gradebook as per How can I change my grade book setup?, or change the assignment type on the assignments so that it is no longer correlated with the one we are editing.
4. Add the Letter Grade Scale to the Assignment Types in the Gradebook
5. Re add the Assignments as per How can I change my grade book setup? and re input the Grades as Letter Grades