The useful idea
The right modernization makes a defined constraint easier to live with or removes it.
Name the constraint
Is the problem slow releases, unreliable hosting, expensive operation or difficulty changing one part of the application? Different problems need different work.
Create a smaller finish line
Choose an increment that can be tested and handed over. Keep the transition understandable for the people operating the existing service.
Know why you are changing
Newer technology is not itself a business result. Compare the improvement against transition effort, operating complexity and the ability of your team to own it.