Tom, usually if there is an upgrade version then you can use that on top of the versions that are detailed in the upgrade version's details on the XAMPP for Windows Home Page.
If no upgrade version is available then it usually means that there are too many configuration file or component changes to be able to implement an upgrade and so a fresh install of the new version should be implemented.
However you can have more than one version of XAMPP on your PC at once but importantly, you will only be able to run one version at a time.
So if you have XAMPP 1.7.1 installed in C:\xampp\ then you could install XAMPP 1.7.2 into, for example, C:\xampp2\ which gives you the best of both worlds especially if you have issues with any of the latest version components like php 5.2 and php 5.3 etc.
I have the past 5 versions of XAMPP installed in this way, then if I need to troubleshoot a particular version, all I need do is use the XAMPP Control Panel associated with that particular version (shortcut to xampp-control.exe in the relevant XAMPP version's installation folder) and everything is available for that version on my PC...
...but do make sure that after closing one version to use another version that you also uninstall any of the components you have installed as an NT Service before closing that version and before starting or opening the next version you want to use.
This is why I never use the Svc boxes in the XCP or install a component in XAMPP as an NT Service unless I need to start XAMPP when ever I reboot my PC, which for me is never as I prefer to start/install stop/uninstall NT Services manually as and when required for testing/troubleshooting only.