People prefer consistency over inconsistency.
When returning to a website we like knowing what we’re going to get.
We create a mental image of what a site looks like. When we arrive and everything looks different it makes us feel uneasy and confused.
We hate searching for the checkout button or the latest news pages.
When updating the look, feel and layout of a website do it in iterations, not in an extreme makeover. Teach returning users. Help them feel more comfortable.
The only time a website makeover is needed is in desperation to fix a problem or attract users. Even then, a big change may do more harm than good.