NEW YORK | Transitioning to a next-generation Internet could be akin to changing the engines on a moving airplane.

Routers and other networking devices will likely need replacing; personal computers could be in store for software upgrades. Headaches could arise given the fact that it won't be possible to simply shut down the entire network for maintenance, with companies, groups and individuals depending on it every day.

And just think of the costs -- potentially billions of dollars.

Advocates of a clean-slate Internet -- a restructuring of the underlying architecture to better handle security, mobility and other emerging needs -- agree that any transition will be difficult.

Consider that the groundwork for the IPv6 system for expanding the pool of Internet addresses was largely completed nearly a decade ago, yet the vast majority of software and hardware today still use the older, more crowded IPv4 technology. The clean-slate initiatives are far more ambitious than that.

But researchers aren't deterred.

"The premise of the clean-slate design is, let's start by saying, `How should it be done?' independent of `Can we retrofit it?" said Andrea Goldsmith, an electrical engineering professor at Stanford. "Once we know what the right thing to do is, then we can say, `Is there an evolutional path?"

One transition scenario is to run a parallel network for applications that truly need the improved functions. People would migrate to the new system over time, the way some are now abandoning the traditional telephone system for Internet-based phones, even as the two networks run side by side.

"There's no such thing as a flag day," said Larry Peterson, chairman of computer science at Princeton. "What happens is that certain services start to take off and attract users, and industry players start to take notice and adapt."

That's not unlike the approach NASA has in mind for extending the Internet into outer space. NASA has started to deploy the Interplanetary Internet so its spacecraft would have a common way of communicating with one another and with mission control.

But because of issues unique to outer space -- such as a planet temporarily blocking a spacecraft signal, or the 15 to 45 minutes it takes a message to reach Mars and back -- NASA can't simply slap on the communications protocols designed for the earthbound Internet. So project researchers have come up with an alternate communications protocol for space, and the two networks hook up through a gateway.

To reduce costs, businesses might buy networking devices that work with both networks -- and they'd do so only when they would have upgraded their systems anyhow.

Some believe the current Internet will never go away, and the fruits of the research could go into improving -- rather than scrapping -- the existing architecture.

"You can't overhaul an international network very easily and expect everyone to jump on it," said Leonard Kleinrock, a UCLA professor who was one of the driving forces in creating the original Internet. "The legacy systems are there. You're not going to get away from it."