Oh I so hate the MS store... you have to be Sherlock Holmes to find information there.
Apparently you can use the key only, check it here: Microsoft Office Online
Tell me what you understand, It is clear than when you have a key you don't need to reinstall. It is not completely clear though if when you purchase they will give you only the key or if the key is part of the download process. Sorry if I made you more confuse...
The Conversion Process:
Follow these steps to convert your Microsoft Office System trial product to perpetual use with no uninstall or reinstall necessary.
Step 1: Get Full product Key and Start Conversion Process
Obtain the Microsoft Office System product online or at any retailer to get a perpetual product key. After you have obtained your full product you have two Options to start the conversion process.
In any of the Microsoft Office trial expiration reminder messages, click Upgrade.
Click the File Tab and then click Help. Next to the trial product name is a button labeled either "Licensing Operations" or "Purchase Product". Clicking "Licensing Operations" pulls down a menu with an option to 'Purchase Product'
Note: To avoid uninstalling the trial if you obtain the full product version at retail, the full product version should be the same version and language as the trial software. For example, if the trial software is 2010 Microsoft Office Home and Student English, the full product version should be 2010 Microsoft Office Home and Student English as well. If the full product version is a different language version, you will have to uninstall the trial and manually install the full product version.
Step 2: Convert Your Trial through Set-Up Wizard or Office Anytime Upgrade
Office Anytime Upgrade will walk you through the process of converting a classically-installed trial suite, also known as MS Install. For standalone applications not purchased as part of a retail suite, or for retail suites labeled as "Click-to-Run", please see the instructions below. The correct user experience – Office Anytime Upgrade or Setup – will be launched based upon the particular trial product you have selected to convert.
In the Setup dialog box, click Enter Product Key if you have purchased a full product key from a retailer or click Buy Online Key if you do not have a full product key and would like to purchase it online.
Step 3: Enter the Product Key
Enter the product key shown on the packaging of the full product obtained at a retail store, or the product key that you obtained online. Click Continue.
Step 4: Accept Microsoft Software License Terms to Install
You will then see the Microsoft Software License Terms (MSLT). Accept this agreement to use the software by selecting "I accept the terms in the License Agreement" and click Next.
After you accept the MSLT, an alert window will open saying that your product is being updated. Another message will open once the software has been set up successfully.
After launching the full product for the first time:You will be prompted to activate it. Unlike the trial activation, during activation of the full product, you will have the option to activate the software either over the Internet or by telephone. The default option is to activate the software over the Internet. Once that is done, the trial is converted to perpetual use with no uninstall or reinstall necessary.