Here's what i did to upgrade my 5.4.2 client to 5.4.8 without having to download half of the internet.
The New Patch will be released this Monday, 26th of January 2016.
The process is simple, you just have to download a 25mb~ zip file containing the new patch files and a new launcher¹ and do this.
Step-by-Step guide:
*it is advised to make a copy of your current game just in case*
1) Get the Zip file containing the files & launcher here:
https://goo.gl/GmtCrX
2) Unzip it, and locate the folders.
3) Copy what's in "Data" into your current² 5.4.2 client's Data folder.
4) Copy what's in "Data/enUS" into your current² 5.4.2 client's Data/enUS folder.
***(if you have something different than enUS, simply rename the files "wow-update-enUS-17xxx.MPQ" to "wow-update-enXX-17xxx.MPQ" XX being the language you have "enGB, enFR for example and also change this line SET locale "enUS" in the Config.wtf file to your language by changing enUS to enXX" )***
5) Copy the WTF folder as it is to your current² 5.4.2 Client's Folder.
6) And finally, copy the launcher along with the other files in the main directory "Scan.dll/wow.mfil" provided in the Zip file.
Links!
The patches & launcher Zip file:
https://goo.gl/GmtCrX
Winrar to Un-zip it:
http://goo.gl/y7hIk6
¹.It's a Launcher from a different server.
².By current i mean the one you're using to play right now, please make a backup.