There are two types of contributions that are involved. First, non-elective employer contributions - these are contributions by the employer, that are made to the account - these can be matching, discretionary bonus payments or mandatory. If you haven't yet "vested" these, (meaning you haven't had enough time in the plan), the employer must transfer
to the new plan an equal amount of cash and equal remaining time to vest these funds. For example, if you have two years remaining to vest on a $1000 employer contribution, then you must be provided an equal or greater benefit in the new employer plan - i.e., no more than 2 years and no less than $1000 in the new plan.
On the other hand, you are always vested in your elective contributions. On a plan termination
you can take these contributions and/or roll them over into any other type of tax qualified account, including the new 403b
plan sponsor your employer has chosen, or an individual IRA. The key is that you must be sure that the roll over is tax qualified. If you don't do that, you can suffer an early withdrawal penalty.