So here is goes what HTC says.. http://www.htc.com/us/support/htc-one/howto/365638.html
It says to restore from a file which seemed a fantastic and easy idea but ALAS! There no restore option in keyboard! WTF?
So what we have to is, convert the htcudb file to a text file and restore with a app called Tweak User Dictionary
So the steps are Consifering you have a HTCUDB back file on your SD
1.Backup of your user dictionary from HTC device. You must have the PersonalDictionary.htcudb file. To get this file go to Settings(on your HTC ROM), and then tap Language & keyboard >Tap HTC Sense Input.>Tap Personal dictionary >Back up to email.
When prompted, tap OK. You have the PersonalDictionary.htcudb file now.
2.Any office suite application.(MS Office 2010 Recommended)
3.User Dictionary Manager(UDM). Make sure you have it installed. You can find it on Google Play.
1. Open the PersonalDictionary.htcudb file using the Word Processor. Use encoding UTF-8.
2. You will notice all your added words separated by "|" symbol. Select all data and do Ctrl+G
3. Go to Replace tab and type "|" (without qoutes ofcourse). Click on "More" button, and then click on "Special" button. Select Manual Line Break option from the drop down menu.
4.Click on REPLACE ALL button. Wait until all words are arranged in one word per line format.
5.Save the filename as wordlist.txt. Move it to your sdcard.
6.Open the TUD application. Click on Import from txt file. type the location and select OK. It will be stuck for 3.4 secs and then it will says *** words adde3d. I added 1012 words.
To avoid all this crap above next, just simple back up with the TUD app and restore it with two clicks.
Click thanks If I have helped you a little.
And here is already addded words for some countries which is useful as well!
Thanks to this thread: http://forum.xda-developers.com/show....php?t=2744848