Prisoners on a type of open-ended sentence in England and Wales are being moved to lower-security prisons, to try to ease pressures in the system.
More than 4,000 indeterminate sentences have been given out
The move is meant to help those serving indeterminate sentences called IPPs get on rehabilitation courses.
The Court of Appeal had ruled they were being unfairly denied the chance to show they were safe for release.
Those serving a minimum term of under three years will be downgraded from Category B to Category C prisons.
Under IPPs - or "indeterminate sentences for public protection" - offenders are given a minimum jail term.
However, the sentences are essentially open-ended, as prisoners must show they do not pose any threat to the community before they can be released - even if they have served their minimum term.
Some 4,000 offenders have been handed these terms since they were introduced in 2005.
All had initially been sent to category B institution or above to reflect their potential danger.
But the Court of Appeal recently ruled that there were not enough resources to allow such prisoners access to the courses they needed - such as anger management and alcohol awareness - to be able to make their case for release to the Parole Board.
Now, instructions issued to prison governors by the Prison Service say the policy of sending IPP prisoners to category B prisons has put "pressure" on the system and is being "revised".
"We will cease to treat IPPs like life sentence prisoners, and instead manage them through the closed estate in the same way as determinate sentence prisoners," the order states.
The instructions say IPP offenders must still never be moved straight into "open" Category D institutions.
A Prison Service spokeswoman said the changes, which came into force on Monday, did not reflect a relaxation of security standards.
But prisoners would be able to "access suitable courses to address their offending behaviour".
The Prison Reform Trust said the move might still not be enough to get the government out of the "mess" it had got itself into.
"Given the huge pressure of overcrowding and budget cutbacks, it is still uncertain whether there will either be enough courses or Parole Board hearings to deal with the hundreds of people currently held beyond their tariff," said director Juliet Lyons.
"Until this unjust sentence is constrained by new legislation or scrapped altogether, this is the fairest way of enabling people trapped in a maze with no exit to gain access to the courses they must take to prove to the Parole Board that they do not pose a risk to the public."
Crime-reduction charity Nacro said the move was "sensible" but more fundamental reform was also required.
Chief executive Paul Cavadino said: "The legislation introducing this sentence was so flawed that the prisons are now flooded with IPP prisoners, many of whom would previously have received relatively short, fixed sentences...
"The root cause of the problem is the badly-framed legislation which created the IPP sentence. The sooner restrictions on the use of these sentences come into force the better."