In DE1 and FR1 region, if you use "12.0-53.13 Standard Edition" or "older version" of the Load balancer(NetScaler VPX), please don't change the plan (e.g.: 50,200Mbps⇔1000, 3000Mbps).
If you should try to change the plan with the above-mentioned condition, the operation would be in error and then, you would not be able to do any operation on the Load balancer(NetScaler VPX).
To avoid any issue or error, you need to upgrade the Load balancer(NetScaler VPX) to the newest version, at first.
As for the upgrade procedure, please refer to the following URL.
Tutorials - Load Balancer - Replacing between different versions
After the upgrade, you can change the plan.