I’m working on interface restiling of a portal.
Users of this portal are not exactly fond of technology and some of them are old people and don’t like changes.
This is the reason why I need some good examples of user’s guide to new interface or some good examples of “soft” transition from one interface to another.
I’m not sure there is a guide for specifically what you’re looking for. You could try this classic article from ALA: http://alistapart.com/article/redesignrealign
Consider what types of changes the community might actually want, and start there. Also think about small things you can do gradually to work towards the bigger change that you imagine.