Need to upgrade a site from 3.07 to 3.53 - what issues might I encounter??
The most common issue when upgrading between PHP 5.6 and 7+ is legacy MySQL functions. Some common MySQL functions such as mysql_query and mysql_error are no longer supported in PHP 7 and must be replaced by their MySQLi equivalents.
We have built a plugin that can help detect these functions called the Legacy MySQL Scanner, which is included with CMSB since version 3.12. This plugin can be used to scan your plugin and website directories for the existence of these legacy functions and will tell you what they should be replaced with. For plugins that have official releases, it can often be easiest to simply update those to the latest version.
This upgrade is made slightly more complex as CMSB 3.07 did not yet support the new MySQLi functions, so the legacy functions can't be changed before upgrading. As kitsguru suggests, if you're able to test out this upgrade locally (or on a separate subdomain on your host) that would help avoid potential downtime for your site.
Good luck! Let me know if you have any more questions.