Urgent: Managed Services Migration Breaking Websites, Need Help!

I'm completely stuck and tearing my hair out trying to migrate several client websites for our 'Website Maintenance & cPanel Management Services' to a new managed services provider. Every single site is breaking post-migration, mostly with database connection errors and weird cPanel configuration issues. This transition to new managed solutions is proving far more difficult than anticipated.

I've tried re-importing databases, checking config files, and even manually adjusting paths, but nothing seems to stick. This is urgent as clients are starting to notice downtime, and I'm running out of ideas on how to fix these persistent issues.

What are the most common, insidious issues that pop up when migrating cPanel accounts between different managed services setups? Are there any specific pre-migration checklists or post-migration scripts I might be missing that address database integrity or server pathing for these complex managed solutions? Any tools or strategies you'd recommend for a smoother transition? Help a brother out please...