It’s pretty simple if you use dynamic material instance.
1.Create One material
2.Add both of your textures between which you want to change over time
3.Add Lerp node and connect it’s alpha to scalar parameter, name scalar parameter since it will be the handle to change between two textures. Here’s simple example
4.Now in construction script of actor create this nodes
5.Now in event graph add this nodes.
6.Add float track to your timeline so that in 3 seconds it will change float value from 0 to 1.
thus whenever you press T, or it at any event’s it will change material smoothly since we feed slider value to scalar parameter over 3 seconds.