The biggest problem in answering this is the definition of 'early' and 'late' lupus. Some patients have very minor symptoms that persist for years and need almost no treatment; others are very sick from the outset of their very first symptoms.
Most people use the term 'early' to denote time, but in lupus the issue is probably one of severity. Clearly, the sooner the disease is recognized, and the sooner treatment is initiated, the less damage will occur. However, early treatment does not necessarily prevent later flares. Patients with SLE follow three types of courses: chronic active (accounting for about half of patient-years), relapsing-remitting, and long-remitting.
There's also the concern about what sort of treatment is indicated. In some cases, that means corticosteroids; in others, immunosuppressives are needed, and in still others, anticoagulants. All have dangers associated with them, and it is a judgment call whether to use them or not in given circumstances. For example, even management of asymptomatic patients with anti-cardiolipin antibody is still controversial.
There is, in fact, so much variability in the treatment of an individual lupus patient that I cannot provide a clear answer about early or late treatment. However, I do not know of any research suggesting that early treatment prevents later serious flares. (For further treatment information, see Dr. Lockshin's In-Depth Topic Review on SLE.)