No - you can change the operating system at any time. The manufacturer may not support the operating system, but you can always put it back to the original operating system if a warranty repair is needed.
There should be an option to create "recovery discs" or something similar. This will create a set of discs to return the laptop back to the original OS.
One thing to note - make sure you have drivers for the new OS - some manufacturer's don't have drivers for other OS's.