You need to confirm the correct license expiration date before adding seats or merging licenses/accounts in WFBS-SVC. This article provides you the formula for computing the correct license expiration date.
When merging two licenses/accounts or adding additional seats in WFBS-SVC, the expiration date is prorated.
Below is the prorated license formula:
(original seats * original days left + newly added seats * newly added seats days left) / (original seats + newly added seats)