First welcome Shahrad, second, never ask for a direct reply in a forum like this. People who help you with the answers will generally help you, because it can help many other members also. And that would not work if they just emailed you the answer. If you like to have the answer on your email, then you can subscribe to the answers to this tread instead. Then it helps you and everyone else.
But to your problem. What you have been doing here is something I have always told everyone NOT TO DO, as it is basically a VERY VERY BIG TASK! The number of tables the item description is linked to is very big and there are a lot more tables where you must do your change before it will work without your overflow error.
I would recommend you either to reconsider your change (maybe just use extended texts for items) or install the Development Toolkit for NAV and use the cross reference functionality here. Also consider that such a big change makes all your future upgrades much harder.