@LockeKosta, I know this is an old post, but you can modify an Input Action at run time, but you have to Flush and Rebuild the Input Mapping Context. Flush being the key part.
For example, if you have an Input Action with a Pulse Trigger. In blueprints get a reference to the Input Action get the array called Triggers from it. Find the Pulse Trigger in the array. From the Pulse Trigger Set Interval to something new(Default value is 1.0 for pulse intervals). Then call Request Rebuild Control Mapping with the Target being the Enhanced Input Local Player Subsystem and the Rebuild Type set to Rebuild with Flush.
The process has to be similar in C++ though I haven’t tried it yet. If I have any success, I’ll share my code here for posterity.