Permalink Structure Change – Help Needed

Hi there,

I, not so long ago, changed my permalink structure from the ‘day and name’ format to /%postname%/

so from http://samplesite.com/2010/11/12/sample-post
to http://samplesite.com/sample-post

I added this to my .htaccess file:

RedirectMatch 301 ^/([0-9]{4})/([0-9]{2})/([0-9]{2})/(.*)$ http://samplesite.com/$4

I would like to now change my current permalink structure to /%category%/%postname%/

So here are my questions:

1. How would I get about doing this?
2. How do I take into account my previous 301 redirects? Or do I even have to?
3. Is this change even recommended?
4. Will there be any resulting issues?