Will date added change if factory reset is done

I never even noticed the date added, but it appears to be when the device was added to the Roku account, so if you add it to a different Roku account the date will reflect the date it was added to that account.  So... yes.

Note that subscriptions and purchase made through Roku are tied to the Roku account, not the device, so if you have subscriptions tied to the account on the device that you want to factory reset you'll lose access to them when you link it to a different account.  If you never subscribe to anything through Roku (recommended), then this won't matter.  If you're not sure what may be linked to what account, check your subscriptions at https://my.roku.com using the email address that's displayed on each device under Settings->System->About.