After each ejaculation then yes you should replace the condom you are using. Thats why its a good idea to have some quite close at hand so u can quickly put on another one. When it comes to pre-ejaculation then I'm not sure if theres any need to change. If you want to be careful then do so also.
Premature ejaculation is still ejaculation you have to change the condom <B><I>EVERY TIME </B></I>you ejaculate. The semen will work its way up the shaft and out of the condom if you don't change it every time.