Migrate wordpress posts + featured images from old theme to new theme and have them show up properly on new theme

Hello, This is for experienced Wordpress / php developers with mysql know how. I am having issues with transferring over a old wordpress theme that has around 500 posts to a new theme. The new theme is on a staging server. I cannot get featured images to import to the new theme from the old theme. The posts are able to be imported, but the file attachments / featured images are not. I have used the Export Featured Images plugin, and when trying to upload the XML file, the upload times out, causes a database error and requires me to set a new user in order to gain access (somehow it is changing or effecting the login). The hosting is on domain.com. In addition, I have tried to use the method as detailed here [login to view URL] The issue here is that when trying to copy the WP_Posts and WP_meta tables to another table, I am getting a syntax error and when I fix that syntax, it creates the table + structure but not the data. Thus, I have not been able to use this method yet. What I need is for the following: 1). Import posts + featured images from the old theme to the new theme. 2). Adjust the thumbnail size in [login to view URL] (or where necessary) so that the featured images look correct when being used in the new theme, as in the old theme. Right now, a few featured images were uploaded, but the thumbnail sizes for the new theme, are greater than the thumbnail sizes for the old theme, thus the images are being distorted as with the old theme the thumbnail sizes seem to be a max width of around 150px, but the new theme has them being set around 325px or so. This needs to be fixed so that the images are not distorted. Please message me for web url's for both sites.