SAN FRANCISCO – Gov. Jerry Brown did not illegally gut an existing ballot initiative to try to get his plan to reduce the state’s prison population before voters in November, the California Supreme Court ruled Monday.

The 6-1 decision all but guarantees that voters will have a chance to weigh in this year on Brown’s changes increasing sentencing credits for adult inmates and allowing earlier parole for nonviolent felons. The plan gives Brown a chance to ease a tough sentencing law he enacted during his first term in office in the 1970s that he has since called a failure and add his stamp to the growing movement for prison reform.

The California Supreme Court said Brown’s changes were consistent with a 2014 law requiring amendments that are “reasonably germane” to the original initiative and mandating a 30-day public comment period for initiative proposals.

“There is no question that the changes the proponents made to this initiative measure were, in certain respects, quite extensive,” Associate Justice Carol Corrigan wrote for the majority. “However, that is their right, so long as the changes are reasonably germane to the original theme, purpose, or subject.”

Brown in May submitted nearly double the number of signatures he needed to qualify his measure for the November ballot, though elections officials have to sign off on the validity of the signatures.

Brown said the changes were needed to help keep the inmate population below the level required by federal judges, and it was too late to start over and collect the signatures needed for a ballot measure this year. The measure would increase sentencing credits for inmates who complete rehabilitation programs and allow nonviolent felons to seek parole after they have completed their base sentences, without enhancements for things such as gang involvement or firearms possession that can add years to a prison term. The changes would mitigate the fixed sentencing law Brown signed during his first term.

“(Californians) will now have a chance to improve public safety by voting to provide incentives so that more people follow the rules, educate themselves, and turn their lives around,” Dan Newman, a spokesman for Brown and other initiative supporters, said in an email.

The California District Attorneys Association sued to block Brown’s initiative, accusing the governor of completely rewriting the original juvenile justice measure to sidestep the normal initiative process and failing to give the public a chance to comment on the changes. Allowing Brown’s changes would encourage other initiative proponents to pull a similar bait-and-switch, it argued.

A lower court agreed with the association and in February blocked Attorney General Kamala Harris from issuing documents that Brown supporters needed to gather signatures. The state Supreme Court overturned that decision Monday.