Good Serps But Pages Need Changing

Yahoo - should i change design

[layer8]

I have been working on a new site design which is sleek.

The problem is my old (really badly designed) pages are coming up in serps at good positions.

I really want to edit the old pages to make them look fresher - providing I don't mess about with it too much am i likely to alter my positions?

This referes only to Yahoo only..thanks in advance!