On March 30, 2020, there will be a new exam for the Juniper Networks Certified Associate, Junos (JNCIA-Junos) certification. The JN0-102 exam will end of life (EOL) on March 29, 2020.
This change reflects an update to the exam. The primary driver for the update was to remove the Networking Fundamentals objective and underlying topics.
Detailed exam information for the new Junos, Associate (JN0-103) exam can be found in the certification tracks where the JNCIA-Junos is the prerequisite: Enterprise Routing and Switching, Service Provider Routing and Switching, and Data Center tracks.
The exam numbers are the particular pool of questions that are taken. Your certification in both cases is the same JNCIA-Junos.
Once you pass any currently authorized exam you will have the JNCIA-Junos certification for 3 years from that date. There is no effect when exam pools are changed.
My experience with vouchers is that they will work on the registration site up until the expiration date you have with that voucher so that should be fine with the exam number change.
Good luck in your preparations.