The already frenzied preparation for the Sept 20 iPhone 5S release date gets a bit more exciting as U.S. retail giant Best Buy ramped up efforts to sweep out as much as iPhone 5 stocks just a few weeks before the anticipated D-Day.

The gadget seller dangles up to 50 per cent price cuts on the soon-to-exit Apple flagship in exchange for working and still pristine-looking iPhone 4 and iPhone 4S. The giant promo runs through the Labor Day weekend, in which iPhone 5 buyers can get discounts of between $100 and $150, respectively for the 16GB and 32GB models.

In theory, the Best Buy credit can still be topped up depending on the conditions of the handsets to be swapped. Those that appear to be coming out-of-the-box during the original purchase date are sure to attract higher cash value, Apple Insider said on its report.

Also, customers cutting into the deal should ensure first that they are eligible for phone upgrades with their respective service providers as all packages come with 2-year contracts from U.S. telcos.

Or they can actually wait, as advised by CNET, adding that the Best Buy promo campaign is obviously the retailer's last minute attempt to clear out its inventory of iPhone 5s short of three weeks prior to the supposed iPhone 5S release date on Sept 20.

There was no mention if credits gained on surrendering the iPhone 4 and 4S can be carried over to a future 5S purchase so the safest alternative is to wait out and take advantage instead of the trade-in deals coming from Apple and online device swapping site Gazelle.

According to recent reports, Apple Stores across the U.S. are set to welcome gadget-swappers shortly after the 5S availability while Gazelle has already commenced on its offer of price-locks for deal hunters with customers able to freeze on a given deal until Oct 15 or weeks after the 5S debut.

It is to be expected that more deals, likely coming from Amazon, Target and Walmart, will emerge in the days and weeks ahead as the 5S release drumbeats get stronger, luring consumers to get a taste first of the iPhone 5 before jumping into the 2013 edition, observers said.

Eleven more days to go and the 5S will become official as Apple is set to unleash the handset along with its rumoured low-cost version, the plastic-wrapped iPhone 5C. Everyone also looks forward in seeing the final build of the iOS 7 but not before the GM edition is let out on Sept 5.

The 5S features to watch for are the revised Home button, which reportedly is now protruding because of the fingerprint sensor embedded underneath while coated with sapphire, a faster A7 processing chip with 64-bit computing capabilities and enhanced Retina display panel that packs the 4-inch screen with some 1.5M pixels.

On its actual Sept 20 release date, the iPhone 5S is reported to become initially available in Japan and the United Sates with key markets around the world to follow in what is desribed as phased global distribution that will go full blast during the 2013 holiday quarter.