Dr Peter Carter said 'properly thought through' reconfigurations would not be opposed - but the union was to 'get more sophisticated' in tackling 'slash and burn' cuts designed only to save money.
Speaking to HSJ, Mr Carter also launched an attack on poor financial management, saying that a minority of poor NHS managers were distorting the NHS's overall financial position.
Signalling a change of direction, Dr Carter told his first RCN annual congress on Monday that 'the RCN can't simply moan or complain'.
'Instead, we have to offer solutions and promote alternatives,' he said. He told HSJ: 'Where it is properly thought through and where it has got good health outcomes we will support hospital closures.'
But reconfigurations to save cash - of which there were 'copious' examples - would not be tolerated, he said. The 'way forward' for the RCN would be for local branches to scrutinise trusts more closely by, for example, monitoring board meetings and getting involved in local overview and scrutiny committees. He said: 'I'll be clear, our performance is disparate on that.'
Closer working with the government was also needed, he said, after the RCN's relationship with the Department of Health plummeted with the heckling of health secretary Patricia Hewitt at last year's congress.
He said: 'Relationships were not good and what we have been doing over the past few months is rebuilding those relationships.'
Dr Carter, chief executive of North West London Mental Health trust for the past 12 years, said nurses were not 'luddites' who were resistant to change.
Yet job losses 'shouldn't really be on the agenda' considering the 14,000 shortfall in nurses predicted by the DoH and revealed in HSJ in January. He said: 'With proper workforce planning what you can do is ring-fence posts and re-deploy people into existing vacancies.'
Good management was key to this - but expertise was often lacking, particularly in commissioning. He said: 'Unfortunately, about 20 per cent [of NHS managers] are not up to the mark and it is that 20 per cent that significantly distort the performance of the other 80 per cent.'
In his speech at congress, Mr Carter said the NHS 'is having the life squeezed out of it by deficit-led cuts and reform overload'. These 'pressures and problems are growing by the day'.
He said: 'The rules of the game have changed. Nurses have had enough. We have been silent too long and we have turned the other cheek too often. We have reached our limit - this far and no further.'
Money had been 'wasted' on too many 'bureaucratic structures', turnaround teams and management consultants, he told delegates.