I'm thinking about moving my site to a new host. From reading on this forum, it seems like it shouldn't affect SERP much, if at all, as long as I keep the same http://mydomain.com. However, what about changing page extension from .aspx to .htm or .php? Will Google consider them as brand new pages? Will I loose much or all ranking? What is the best way to redirect deep links to corresponding sub pages? Thanks a lot. My site is really being picked up by Google and I see more traffic and higher ranking every day, so don't want to screw up.
You need to implement 301 redirects for all your extension changes. Otherwise, yes, you will lose your rankings.
you should keep your URLs exactly as they are. Even if you implemente a 301 it does normally take some time to transfer the popularity to the new URLs and it might not transfer it all. I´d avoid as much as possible a change in URLs.
Implementing or not 301 redirect (which will help ONLY to redirect users going to the old URL into the new URL) it will hurt your page ranking IF you have a PR for the page, example yoursite.com/yourdir/index.aspx , instead if you have PR on yoursite.com/yourdir/ it should not hurt (at least thats what i think) but lets wait someone more expert and that has had life experience with this to better tell.
I'll try not to touch the existing URL just to be safe. On the other hand, even though traffic is really picking up, it is still a new site so if there is a better reason to do a complete overhaul, it probably makes sense to do it now. Got to make a decision soon Thanks for the suggestions.