Dart Tag will be replaced by Dart Charge which is managed by a new service provider. Apparently they will contact you with setails how to arrange a seamless transition to the new system, good luck!
see here,
https://www.gov.uk/government/news/d...remote-payment
As a new (non business) user I registered our three cars. Quick process just put in the cars reg and it confirms make and model etc. I chose auto top up as we don't cross to the dark side very often.