g1smd - 8:31 am on May 17, 2012 (gmt 0)
If you keep same domain name and same page URLs, including extension, Google will have little idea that anything has changed.
If you change the page URLs or extensions and do not redirect, you're in big trouble (however, see below as you don't have to have new extensions at all).
Is the site dynamic or static?
The move from IIS to Apache isn't the real issue here. Is the site moving from ASP to PHP technology during this change? If so, then you can still use the old URLs and extensions with the new system. There's just a few changes to make to the site configuration to make this work.
If you're already using extenionless URLs you'll have the least amount of work to do when you migrate.