You can do either way in my opinion. From what I did with my application, I separated my different job titles for the same company (same work and NOC, but was promoted to two other job titles) and uploaded the reference letter from the company in all three fields for the three different job titles. I've also included an explanation letter, explaining everything in detail since the reference letter from employer had only the current job title. In the explanation letter, I've mentioned all my job titles and have shown the promotion letters/pay stubs having the job title as proofs. My application is approved.