The box will continue to work as a retail YouView box. You may have to do a maintenance reset for it to work on your new providers network. You will continue to get updates via TalkTalk to the box when they are scheduled. You will though lose all access to TalkTalk services.
Normally it's yours, but if it has ever been replaced then TalkTalk can ask for it back under their terms and conditions, but I don't think they do in practice. If they do ask for it back they will send you a returns bag, but it may be worth querying why it's been sent when the box was advertised as free.
My contract ended today, and for some reason now my "newer" Youview box (out of the 2 I've got) has developed a 4:3 size screen, low resolution and no sound. Would this be talktalk bricking/blocking the box? I thought that I could continue to use it as normal (obliviously minus the talktalk on demand/subscription channels)
weirdly my old Youview box still works, but HD channels don't....Although it isn't connected to the Internet yet. Hoping an Internet connection gets me back the HD channels.
TalkTalk have no means of disrupting any boxes.
Have you changed supplier? If so a soft reset may help as this could be due to a residual memory of TalkTalk settings. Only try the maintenance reset below if you have an internet connection as the box requires this to set up properly.
To do this hold the power button on the front for around 10 seconds and the box should restart. Alternatively, you can turn off from the back and power back on.
If this doesn’t work then try a YouView Maintenance Reset (https://community.talktalk.co.uk/t5/Articles/How-to-find-maintenance-mode/ta-p/1429347)?
Use the Factory Reset keep recording option.
You will then need to reset your personal settings (such as parental controls / RF loop through / Surround Sound etc.) and also reset your scheduled recordings (might be worth taking a note of future recordings before doing this so you don’t miss any).
Hope this helps