Check out the book "Working Effectively with Legacy Code", by Michael Feathers[0].
I believe the basic approach is to write tests to capture the current behaviour at the system boundaries - for a web application, this might take the form of automated end-to-end tests (Selenium WebDriver) - then, progressively refactor and unit test components and code paths. By the end of the process, you'll end up with a comprehensive regression suite, giving developers the confidence to make changes with impunity - whether that's refactoring to eliminate more technical debt and speed up development, or adding features to fulfill business needs.
This way, you can take a gradual, iterative approach to cleaning up the system, which should boost morale (a little bit of progress made every iteration), and minimises risk (you're not replacing an entire system at once).
I've used this approach to rewrite a Node.js API that was tightly coupled to MongoDB, and migrated it to PostgreSQL.
I believe the basic approach is to write tests to capture the current behaviour at the system boundaries - for a web application, this might take the form of automated end-to-end tests (Selenium WebDriver) - then, progressively refactor and unit test components and code paths. By the end of the process, you'll end up with a comprehensive regression suite, giving developers the confidence to make changes with impunity - whether that's refactoring to eliminate more technical debt and speed up development, or adding features to fulfill business needs.
This way, you can take a gradual, iterative approach to cleaning up the system, which should boost morale (a little bit of progress made every iteration), and minimises risk (you're not replacing an entire system at once).
I've used this approach to rewrite a Node.js API that was tightly coupled to MongoDB, and migrated it to PostgreSQL.
[0] https://www.amazon.com/Working-Effectively-Legacy-Michael-Fe...