After separation of service, distributions from the plan maintained by the employer from which you separate from service in or after the year you reach age 55 will be eligible for the age-55 penalty exception. However, based on what you said, the plan from which you propose to take the distribution is maintained by a former employer, not your current employer, so this exception would not apply.
If your current employer has a 401(k) or 403(b) plan to which you could do a rollover from the 403(b) at the former employer, you could then use the age-55 exception on a distribution from your current employer's plan after separation from service at your current employer.
