This is the biggest mistake i have done ,my site has been steady at page 1 in 10 search result from 2m recently,unfortunately i was too noob because i thought, it will rank first result in page 1 ,but it appears few days after i change it,the site drop 2 page and my traffic get a low from this week.
this is a real mistake but no problem you go to your ftp and replace your old url than after some days your site pr reverse its true but first you change your proper address
This is because non-www and www pages are classed as two different pages. If you've done the .htaccess file correctly then you should have done a 301 redirect rule. Just give Google a bit of time and all your old rankings SHOULD be back to where they were. Best practice really should be to standardise on www or non-www when you create the site so this kind of problem doesn't occur.
Make sure you do canonical 301 redirect through .hhtaccess. This problem will be resolved and all link juice will be restored to single URL.
That is a big mistake, indeed. You were supposed to use 301 redirect to avoid traffic and backlink loss.