Well, I have a website which run on Drupal cms. When i installed a few years ago i didnt activated sef url. but still google listed and it and it has pr4 but not much traffic. so atm urls look like this www.mysite.com/?q=node/48 is it worth now to activate the sef url and make cleaner urls and better title tag? cuz all indexed pages on google will throw 404 and i am afraid i gonna loose rank..
Yeh but the problem is google has already indexed like 500+ pages which all are non sef! so if i change urls to sef it may effect the ranking? even tough i use 301 redirect.
Then, I suggest you to do a manual 301 redirection on all 500+ pages. It is the only best way not to get the error page. There are some webmasters forced to do thousands 301 redirection manually.