hey calamityjen ,

Depending on your need, you can increase the capacity of an existing premium capacity - that would give you the "merge" level of capacity.  There is no "merge" capability of taking two separate capacities and putting them together.

So, if you ramp up one of your capacities, you will have to go through and reset all of your workspaces to the one ramped up capacity.  This inclused reworking all of your gateway usage.  Which, if you are modifying  the gateway, you will have to have every report that uses that gateway to be re-worked to point to the gateway on the ramped up capacity.

And, unfortunately, there is no "gateway transfer" or "capacity transfer" capability to do this in one shot.