I have just completed my 8 precepts last Saturday at BW Monastery.
One of the fellow student raised this question. “While it is said in the sutras that by observing 8 precepts for a day, the merits generated from such an act will allow an individual to guarantee a rebirth in the happy realms. However, if we were to continue to indulge in our other non-virtuous acts for other days, will this eliminate our chance of that happy rebirth?”
I will attempt to view it based on the concepts of Karma.
By completing the 8 precepts, the merits are definitely generated and one will enjoy the fruits of these merits (in this case, a rebirth in the happy realms).
Going by the rule * Karma Is Definite– Reaping What Is Sown, the Karma will not be “eliminated”.
Similarly, by indulging in other non-virtuous acts for the other days, one is creating new Karma that will eventually ripen and cause suffering. Depending on the weight of these new Karma, they could ripen earlier if it was a serious action.
However, I read from Master Ri-Chang’s lamrim commentary (track 122b) that there are cases in which there is destruction of the roots of virtue ( 善根).
Still, there are many wrongs other than anger that combine both production of a terrible fruition and destruction of the roots of virtue:
wrong views that deny cause and effect; abandoning the sublime teachings; generating pride in relation to bodhisattvas, gurus, and the like and thus terribly belittling them; and so forth. You can know of these from the 《Compendium of Trainings》.
The karma created from the 8 precepts will not be eliminated by the other non-virtuous actions, unless those non-virtuous actions are done as a regret or repentance of the virtuous act of 8 precepts.
However, whether the positive karma from the 8 precepts will take effect first or the negative karma from the non-virtuous actions will take effect first, depends on which is more weighty. The weight of the 8 precepts are very heavy, so if a person continues to live his/her life as he/she has been living (I am assuming that a person who has this desire to observe 8 precepts wouldn’t be a very evil person), then the non-virtuous actions that he usually commit through habitual tendencies(e.g. killing insects, telling white lies etc) will not take effect first. So this person will definitely gain a rebirth in the happy realms in his/her next and even many future lives.
Of course if this person has a major change in life’s direction and start committing very weighty negative actions such as the five sins of immediate retribution, then this will take effect first, and the rebirth in happy realms will have to wait.