I just upgraded from free to custom plan, using Android remote camera. Strange thing happened, camera started saving images each 15s instead of 2mins immediately after upgrade. 15s is what I ordered as a maximum. Is there a possibility to change it back remotely before the camera consumes all the data on the remote mobile phone before I can get there?

I just checked it and found the cause. Your ordered a service plan with max upload frequency of 1 image per 15 seconds. If you use a regular IP camera / DVR, then you will need to re-configure it to change the upload frequency. CameraFTP server cannot access/control your device. 

If you use CameraFTP's Mobile Security Camera app, then it is different. The app can automatically check for service plan changes. If the upload frequency is too high for you, you can change your service plan online: order a new plan with lower upload frequency; then go to My Cameras page, click the Settings icon to edit the camera properties and then change the camera license; then go to My Account page, click Manage Subscription and cancel the old subscription.