I have an Inspiron 620 (mid 2011) - on your support site it indicates:
Dell has not confirmed that this product can upgrade to Windows 10. Some drivers, firmware, applications, and connected devices may not work as you would expect.
For systems that fall into this category, does this mean that Dell will never confirm that this system can upgrade to Windows 10, or just that you haven't had a chance to test this particular model, but you eventually will?