THE OBAMA administration this week announced that it is again rewriting some of the Affordable Care Act’s rules, offering yet another extension for noncompliant health-insurance plans. The decision is not justifiable: It’s time to implement this law fully.

Senior administration officials insist that they simply want to smooth out the transition, particularly for those who want to keep old plans that don’t have the consumer protections the law now demands. The administration is giving people who still have plans they bought in the old individual insurance market at least three more years before they must move into the new system. It is offering similar treatment to people in small-group plans. In total, this may allow about 1.5 million people to stay in the old system, whose skimpy plans and coverage cutoffs the ACA was designed to eliminate. With time, officials say, people will voluntarily leave their old plans and the legacy market will disappear.

Some health-care experts aren’t as sanguine. The new system needs lots of customers to achieve a balance between the healthy and the sick, the former offsetting the costs of the latter and keeping premiums down. Any shift that might discourage enrollments is worrisome.

Administration officials point out that their latest extension will probably enable a relatively small number of people to keep their noncompliant plans. But another rule rewrite announced this week suggests that the administration believes there is reason for concern: Federal officials are bolstering the ACA’s mechanisms to stabilize the new system, and documents show that the extension’s possible effects are the reason.

The big danger is that Obamacare will still look shaky a few years from now, even if all the guards against instability and premium hikes work and then phase out, as planned. If insurers worry that the system still appears unstable, that could drive up premiums and deter enrollment. Things could turn out much better than that bad-case scenario. But the administration’s first instinct should be to get as many people into the system as possible, as early as possible. At the least, its latest move adds to the confusion about what protections people have. Only some compelling policy rationale should override these concerns.

In this case, there isn’t one. is working better, so the technical barriers to pushing people into new plans aren’t as high. The law, meanwhile, attempts to establish a norm that all Americans should have coverage of a certain quality and comprehensiveness. In general and over time, this is good for them and for the system as a whole. There’s no reason, other than avoiding another wave of cancellation notices before this year’s elections, to delay the transition again.