Well assuming that the change from 50 to 25 will make it twice as slow, AND entire line features are affected, you can set the scale cost factor to be 2.0 You can select the affected line features and then load them as barriers. If the slow down happens to start and/or end in the middle of a road then you will have to create an appropriate line feature. The slow down should still be 2.0
If this is a permanent change then you should consider updating the values in the network dataset and rebuilding it.
Jay Sandhu