I am trying to upgrade one of our clients production site http://www.spsmw.org/ from 4.9.2 to anything current, and have had little luck. I have been able to get the test site: http://sptest.galvintest.com/Default.aspx to 4.9.5 without a problem, but anything higher fails, or leaves the site unusable. I inherited the project from a former coworker and has such don't know all the details on how the site was put together.

Currently the test site is 5.5.1 and it throws javascript errors, and the navigation disappeared. I left it at a broken state so hopefully someone might be able to point me in the direction of what is broken so it can be fixed, or changed so it will upgrade correctly.

I have tried the following upgrade paths:
4.9.2 -> 5.5 (no upgrade errors but site nav broken, and lost of js errors)
4.9.2 -> 5.3 upgrade errors
4.9.2 -> 4.9.5 -> any of the above (same as 4.9.2 - > 5.5.1)


any thoughts, ideas, prayers to use?

The person who created the site has placed custom styles in the default.css
Therefore, you'll need to compare and merge the old default.css with the new one... or put the old default.css into your portal.css to keep it safe from upgrades.

The script errors and menu issues typically happen on first start after upgrading. You have to recycle the app pool and clear your cache fully after the upgrade and then see if it settles down. Your demo site isn't throwing any errors at the moment.

All of the above should sort it, and then you can start looking at whether any modules need upgrading.
Regards
Rob