I am also going through this right now and essentially got everything moved over only to find basic functions (shipping and payment modules) are now missing from base Prestashop install, in addition to to other finite issues. The theme developer should definitely help you with the install and issues of the theme. That's my common experience buying 6 or 7 themes. But like the others said, the new store should have been put into a subfolder and tested prior to going live. If you cant get the help you need here, you can always go to Upwork and hire Prestashop experts for $15-45/hour or a flat fee based on job required. I am now deciding if I am going to leave Prestashop to Shopify based on the cost on getting my store updated to 1.7.72.
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I am also going through this right now and essentially got everything moved over only to find now basic functions (shipping and payment modules) are missing from base Prestashop install, in addition to to other finite issues. The theme developer should definitely help you with the install and issues of the theme. That's my common experience buying 6 or 7 themes. But like the others said, the new store should have been put into a subfolder and tested prior to going live. If you cant get the help you need here, you can always go to Upwork and hire Prestashop experts for $15-45/hour or a flat fee based on job required. I am now deciding if I am going to leave Prestashop to Shopify based on the cost on getting my store updated to 1.7.72.