Switching to friendly urls on fully indexed sites

[Ur23]

I am currently ranking a site - an online shop - 3 years old domain, 118,000 pages indexed. I am planning to rank subpages of this site, but there is one problem - it has a really awful internal links format (something like _category/?idc=id_63&order=2&page=1&producer_id=0&record_row=long&lang).

Now, there has recently been an update of this site's CMS allowing an easy switch to friendly links.
I thought it would be of great benefit to have the address in nicer format (containing ranked keywords). So far this site doesn't rank much on it's subpages anyway...

But still, it seems quite risky to change those links. Won't Google do something strange?

[ecmedia]

I would say do not touch the old pages since Google will see that you have deleted thousands of old pages and added thousands of new pages. Simply not a good idea to cause this degree of change.

If possible, try to use the new format for new pages.

[buckworks]

Would you be able to set up 301 redirects from the old ugly URLs to the equivalent new ones?

[tedster]

You may find help in our Hot Topics [webmasterworld.com] section, which is always pinned to the top of this forum's index page. In particular, try this thread: "Changing URL Structure [webmasterworld.com] - take on tasks in a logical order"