On Sept. 21, if you believe the rumors, the next iPhone will debut. Along with it, millions of Apple fans will celebrate by taking their well-worn place in line at one of the many Apple stores around the world to get their hands on the newest smartphone.

We already know that the sixth-generation iPhone will be radically different than both the iPhone 4S and 4. Still, is this the time to upgrade? The answer to that question, of course, is a personal one and starts with the type of phone you already have.

Wonder what the next iPhone may look like? Be sure to check out this new video that purportedly shows the sixth-generation iPhone, and was first posted by Macotakara.

If you have a non-iPhone:

For customers who have never purchased an iPhone, or haven’t purchased one in a while, now is the perfect time to join or rejoin the iOS universe. With the next iPhone, you’ll experience iOS 6, Apple’s latest OS for mobile. This iPhone release is largely expected to include near field communication (NFC) capabilities, a larger screen, and for the first time, 4G/LTE connectivity.

For a full list of iOS 6 features and limitations by phone, click here.

If you have an iPhone 4S:

For the millions of customers who purchased the iPhone 4S last fall, you have a very difficult decision to make. Unfortunately, your carrier will play a large role in making this decision since they control your contract. Luckily, carriers have been very aggressive as of late in tweaking user contracts each time a new iPhone is released.

In other words, if your carrier allows you to upgrade without penalty, purchasing the new iPhone is an easy decision. Again, the new iPhone is a huge update.

To see what your contract says, be sure to check your account online AFTER Apple announces the iPhone launch. At that point, AT&T or Verizon Wireless, for example, may shift your phone’s renewal date.

Even if your phone cannot be upgraded, you may be in luck. Check the renewal dates on other lines on your plan. If one of those is upgradeable, you may use that as your upgrade. In that case, your original phone upgrade date will shift to the other line.

If you have an iPhone 4:

This one is tricky and largely depends on when you made your purchase. If your iPhone 4 is no longer on contract, the time is right to upgrade since many of the features of iOS 6 won’t be compatible on the iPhone 4. These include: flyover and turn-by-turn navigation, Siri, and FaceTime over a cellular network.

If you have an iPhone 3GS:

Most likely, the arrival of the next iPhone will mean the end of the line for Apple’s least expensive handset, which was first released in 2009, but is still available through Apple and AT&T. Although the iPhone 3GS will support iOS 6, it will lack many of its most promising new features. These include those features mentioned above, plus Safari’s Offline Reading List.

One final note:

If price is a key factor on whether you’ll upgrade, the arrival of the sixth-generation iPhone will probably mean new discounts on previous models. For example, the iPhone 4S will likely be available in September for $99, while Apple will probably drop the price of the iPhone 4 to next to nothing with a two-year contract.

Therefore, even if you can’t afford the next iPhone, its launch could mean a new handset for you!

Bottom line:

We know that many of our readers will upgrade to the next iPhone regardless of their actual needs and/or contract limitations. For the others, we hope this guide helps you better prepare for September. We’ll certainly update this guide once the next iPhone is officially unveiled by Apple.

One final note and this is really a no-brainer: Do not upgrade your smartphone from now until Apple’s iPhone announcement. Doing so will guarantee that you won’t be able to upgrade to the next iPhone this year.

As a reminder, Apple is largely expected to announce the new iPhone in September. At the same time, Cupertino could launch a first-generation iPad mini, and new versions of the iPod touch and iPod nano.