I found this excellent picture over at digg that showed two very different tactics in determining how you should feel about tweaking your logo.

Remembering the Target
The Coke v Pepsi battle is one of marketing’s finest case studies:
- Coca Cola (aka the market leader)
Just think of coke’s Xmas adverts; traditional values rule for the Coke marketing team. They are secure with the biggest market share. People buy Coke because its not changed for a hundred years. Mums buy Coke for the family - Pepsi (aka the challenger)
Everything that Coke is they attack. The flout tradition and push directly for a young audience desperate to try new things. They will try anything to get some extra market share. Kids buy Pepsi for themselves
So what does this mean to me?
If your the market leader, never change your logo. People buy from market leaders because they feel safe.
If your not the market leader find ways to be different to them. People wont remember your logo changes but will remember what makes your products and services better.

I guess it depends on how you wish for your company to come across. Other market leading brands have logos that have evolved. For Coca Cola, a major part of their branding is their heritage, and how things haven’t changed. Some market leaders have changed their brands, as they want to come across as a company that is staying current and I guess more dynamic. A lot of the time, the changes are subtle – the Shell logo is a good example – a similar style for the last 60 years, without now looking retro. Coke WANT to look retro, which might work for soft drinks, but wouldn’t work for a technology company.
Check out this old Tesco logo. That’s how I remember them, like Lidl only ten times worse. Amazing how they have turned it around.
http://farm1.static.flickr.com/129/318090340_abf181def1.jpg?v=0