This is explained in the novel 2010: Odyssey Two. Jupiter's mass hasn't changed.
It follows that there's no need to reconfigure the solar system to accommodate it:
Do you have any idea what happened?’
‘Only that Jupiter’s turned into a sun.’
‘I always thought it was much too small for that. Didn’t someone once call Jupiter “the sun that failed”?’
‘That’s true,’ said Vasili, ‘Jupiter is too small for fusion to start - unaided.’
‘You mean, we’ve just seen an example of astronomical engineering?’
‘Undoubtedly. Now we know what Zagadka was up to.’
‘How did it do the trick? If you were given the contract, Vasili, how would you ignite Jupiter?’
Vasili thought for a minute, then shrugged wryly. ‘I’m only a theoretical astronomer - I don’t have much experience in this line of business. But let’s see… Well, if I’m not allowed to add about ten Jupiter masses, or change the gravitational constant, I suppose I’ll have to make the planet denser - hmm, that’s an idea…’
His voice trailed off into silence; everyone waited patiently, eyes flickering from time to time to the viewing screens.
The star that had been Jupiter seemed to have settled down after its explosive birth; it was now a dazzling point of light, almost equal to the real Sun in apparent brilliance.
‘I’m just thinking out loud - but it might be done this way. Jupiter is - was - mostly hydrogen. If a large percentage could be converted into much denser material - who knows, even neutron matter? - that would drop down to the core. Maybe that’s what the billions of Zagadkas were doing with all the gas they were sucking in. Nucleosynthesis - building up higher elements from pure hydrogen. That would be a trick worth knowing! No more shortage of any metal - gold as cheap as aluminium!’
There are some consequences on Earth though, mostly relating to the addition of an extra small sun to the sky:
the end of night had vastly extended the scope of human activity,
especially in the less-developed countries. Everywhere, the need for
artificial lighting had been substantially reduced, with resulting
huge savings in electrical power.
Many nocturnal creatures had been seriously affected, while others had
managed to adapt. The Pacific grunion, whose celebrated mating pattern
was locked to high tides and moonless nights, was in grave trouble,
and seemed to be heading for rapid extinction.