Harry and Mr Weasley only find out about the change in the time of Harry's hearing because of Perkins, who works in Mr Weasley's office. Their conversation gives the impression that it was pure coincidence that Harry and Mr Weasley came in early and that Perkins was able to find them.
"Ah, Harry, this is Perkins.”
A stooped, timid-looking old wizard with fluffy white hair had just entered the room, panting.
“Oh Arthur!” he said desperately, without looking at Harry. “Thank goodness, I didn’t know what to do for the best, whether to wait here for you or not, I’ve just sent an owl to your home but you’ve obviously missed it - an urgent message came ten minutes ago -”
“I know about the regurgitating toilet,” said Mr. Weasley.
“No, no, it’s not the toilet, it’s the Potter boy’s hearing - they’ve changed the time and venue - it starts at eight o’clock now and it’s down in old Courtroom Ten."
“Down in old - but they told me - Merlin’s beard -”
Mr. Weasley looked at his watch, let out a yelp, and leapt from his chair.
“Quick, Harry, we should have been there five minutes ago!”
(Order of the Phoenix, Chapter 7, The Ministry of Magic).
What isn't clear is how Perkins might've found out about the change in time and location. The only communication seems to have been from the Wizengamot to Harry by owl.
“Sorry,” said Harry nervously. “I-I didn’t know the time had changed.”
“That is not the Wizengamot’s fault,” said the voice. “An owl was sent to you this morning. Take your seat.”
(Order of the Phoenix, Chapter 8, The Hearing).
Considering that the whole point of the exercise of moving the hearing was to try and get Harry to miss his trial and be expelled automatically it seems strange that Perkins would be privy to this information. I'd have thought that the true trial time would be closely guarded secret within the Ministry.
Perkins clearly didn't see the owl that was sent to Grimmauld Place. So how could he have possibly known about the change in time and location?