When a new public pension plan is introduced, current insured workers covered by occupational pension plans have to face the dilemma between staying in the current play and joining the new public pension plan. This paper tries to establish a theoretical model to analyze how the individual makes the choice decision. We would like to find out what factors would influence individual decisions, what type of current insured workers would choose to stay and what type would choose to join the new plan. The study suggests both age and wage levels are the most important factors. Generally specking, the probability of an elder worker joining the new plan is relatively higher. If the current insured worker has a higher expected probability of early retirement, he is more likely to stay in the current plan now and will the join the new plan after early retirement. If Taiwan National Pension Plan was implemented, the current insured workers would face this choice problem. Using the examples of both Labor Insurance and Public-Sector Employee Insurance, the simulation analysis suggests standard wage, full benefit, and the age of receiving benefit are important variables influencing the probability of the current insure worker joining the new plan. The effect of the age of receiving benefit is the most significant one and it is negative. On the other hand, both standard wage and full benefit have positive effects. These results obtained by both theoretical model and simulation analysis should be able to provide useful implications to policy makers.