But since Tether Limited are the ones who back the tokens with real dollars, they can effectively unilaterally pressure anyone to upgrade the client. Those who don't upgrade and accept stolen tethers will be left with worthless unredeemable tokens.
Read this lawsuit: Tether only has $50 mill and their wires have been frozen by Wells Fargo since March 2017. So the remaining $600 mill tokens are backed by nothing.
That was filed back in April when there were only $50 million Tether in existence: https://coinmarketcap.com/currencies/tether/ Also, the money that's supposedly backing Tether is in Taiwanese banks, not Wells Fargo, and their problem with Wells Fargo should only affect international wires.