As a safety precaution I would recommend taking a backup of all the templates where you have customised for your own use. The test them on a machine with Office 2010. This way you could make sure you dont lose anything prior to upgrading. Testing them on another machine with Office 2010 will NOT have any detrimental effect on that machine. However what you will need to do is make sure that your templates are called something unique in order not to corrupt any templates on the test machine.
Hope this helps.
Good luck.
Tony