The biggest difference between the WidowMaker and the Premier Widow is the actual sail. The Widow is ripstop nylon, the WidowMaker ripstop Poly (Icarex). This is the main cause for the weight difference. I wouldn't bother changing out the spars as that would only lighten up the kite a small fraction which wouldn't really do anything for you. The Widow comes framed in Skyshark P-Series rods anyways so it is already super light. The only advantage you would probably get is if you go to PT Tapered rods, this would give you slightly more flex towards the ends of the rods which may help a little.
The Widow is an awesome kite to fly anyways. Other than the extra weight, I found nothing that the WidowMaker could do that the Widow couldn't. Both of them trick exceptionally well, are super stable, track excellent, etc... The advantage of the WidowMaker is that it could fly down to 3-4 mph, the Widow's minimum wind was around 5 mph. To change the frame out, you would be looking at appx. $80-$90. I would highly suggest holding onto that cash and using it in the swap meet section to purchase another high end kite - if you wanted to. IMO, that would be a much more wiser way to spend the money instead of trying to modify/improve an already great kite.
If you are really into modifications, do a quick search on this forum for a topic about adding a leech line, that would be much less expensive to do than a re-frame and would probably be more noticeable.
This opinion is obviously worth exactly what you paid for it.