Strategies/thoughts for project migration from WAR to Executable JAR with Spring Boot?

Question

Java

Hello,
share your ideas, strategies for the best step-by-step migration of a regular Java Web App C with Jersey without web.xml, which is then deployed to Tomcat in Spring Boot 2.
The fact is that the project is large enough and we will not get the resources to take it and transfer it at once on Spring Boot. It was decided to try to migrate in parts. At the same time, we are constantly working on this project and adding new features.
How would you do it or do it under such conditions?
Thank you!

Answer 1

and what exactly are the errors after replacing project/packaging with jar and adding maven-dependency-plugin maven-jar-plugin and onejar-maven-plugin
in the pome (except size) ?