There is no provision in the current rules for it to change automatically, so it would need to go through another consultation etc.
But it is unlikely to change anytime soon after already asking people to upgrade. That is the reason why taxis are exempt from the diesel Euro 6 requirement. Many were already forced to upgrade to Euro 5 after 2012, and so will still be able to use those vehicles until as late as 2029 depending on when they bought them. Although incentives and savings make it unlikely any will last that long.
It would also be political suicide for either major party to attempt such a change. The ULEZ was a Tory policy with cross party support, but if they or Labour proposed making people upgrade twice in a short period the other will make that their entire campaign. The ULEZ would need to have no effect and problems worsen for anyone to consider something like that. And even then other actions are more likely.
If you want to guess: next year when the ULEZ starts the Euro 3 requirement for bikes will be thirteen years old, and the Euro 4 requirement for petrol cars will be fifteen years old. Taxis licenses also have a 15 year lifespan.
When Euro 4 for bikes is thriteen it will be 2030, although by then Euro 5 will be ten years old. Also in 2030, Euro 5 for petrol cars will be twenty and Euro 6 will be fifteen years old.
If they do plan to roll it on all that seems to indicate a rough average of the early 2030s, and them probably skipping a generation too.