Writing .htaccess files is something of an arcane art. The problem discussed previously was to do with not having the obligatory /. However, no sooner had the redirect seemed to be working perfectly when the whole site disappeared under the weight of a 500 error (which is being worke don at this moment.)

First stop on looking for how to switch files to a new location without losing search engine ranking is to use a redirect 301, the syntax for which is on http://httpd.apache.org/docs/2.2/mod/mod_alias.html#redirect

However, to make it clear why and how to do this, there was a fair bit of useful help in more comprehensible English on http://www.tamingthebeast.net/ as well as some (as yet untested but apparently good) Search Engine Optimisation advice.