Plant is never “locked” into bloom… once you increase dark time to a sufficient point, it will take several days before the initial “early-bloom stretch” period, but at any time, if you switch the lighting back to vegetative hours, it will revert back to a vegetative state.
Solid plan is to switch lights to 12 on/12 off. 14-20 days later the stretch is generally complete and small buds are beginning to form. If you intend to do any clipping/manicuring of the plant, best to do it before the switch and/or just after flowers begin forming but after plant has stopped stretch (generally around day 16-18 on strains that finish in 10-11 weeks from switch.
Oh and this is all only my opinions and perceived experience and in no way should be viewed as fact.