Override map texture

Let’s say a map uses texture x.vtf and I was to override it to be y.vtf. How can it be done?

[editline]19th May 2014[/editline]

Without forcing a new map download ofc

Here’s a tutorial on how to do it:

https://dl.dropboxusercontent.com/u/26074909/tutoring/map_textures/replacing_textures_in_realtime.lua.html

Also, I had a question regarding scaling textures in real-time in the event you replace a low-quality large-scaled texture with something HD and thus would need to scale smaller:
Question: http://forum.facepunch.com/showthread.php?t=1348923&p=44754354&viewfull=1#post44754354
Answer: http://forum.facepunch.com/showthread.php?t=1348923&p=44756870&viewfull=1#post44756870