How to update media url in the new site ?

Hi guys ,
Im new on this forum and i will appreciate your help .
Im re-designing a wp website from the first steps im stuck .
I tried to make a cope and create a subdomain so i can work on that , but was imposible to create a cope because the ftp took ages ( 1.5 GB ) .

Now i have export all the post from wordpress and import them into the one im working.

I can see that images in the posts have the URL of the main site . im wondering after i finish the work and transfer to the domain , images ulrs wont work.

This website have around 1300 post and will be imposible to go to each of them and import and update the links .

Do you guys know any easy way for that ?

Thanks in advance