I am new in the adsense world, I just installed a php script that comes with already built-in adsense from the script publisher, now I want to open an account for adsense ads and submit my website (the script) for review. My question is, should I remove all the built in adsense in advance so that google see it clean, or it does not matter I just want to replace the ads rather than remove and the re-submit new ones from mine !
yes you should remove author adsense code, otherwise he will get free money from your site. If you applying adsense with that website(already have adsense in it) you will confusing google adsense team..
Just apply for adsense. Sometimes (infact most of the times) they accept if your site meets their quality thresholds.
dont confuse the adsense team,its good that you remove the older ones and then put the new one to avoud confusion although replacement also works