Symptoms
There is no option to upgrade the order to an existing subscription, we can only see an option to upgrade to a new subscription.
Cause
Partial upgrade in UX1 still has some limitations.
For example, it will not work if both subscriptions expire on the same date.
Quote from related documentation page:
Note: The Partial upgrade wizard displays only those existing destination subscriptions that have their expiration date later than the one of the source subscription, have the same Microsoft tenant as the source subscription, are not in the cancellation window, and do not have any pending orders. The Next button will become active at the first step of the wizard only if the number of units selected to be moved is more than the current number of units in the existing destination subscription.
Resolution
If the above limitations are blocking you from using Partial upgrade feature in UX1, you may use Classic panel (Partial Upgrade Tool) instead:
If for some reason you cannot do it neither via UX1, nor via Classic panel, please report the issue to Cloudblue Technical Support.
In time-sensitive cases you may perform the partial upgrade directly on Microsoft side and contact Cloudblue Technical Support. We will sync the licenses from Microsoft to CBC side.