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what i have found is that declarative pipelines facilitate migration quite well, since similar projects can use similar Jenkinsfiles (and you can put shared Jenkinsfile functionality in libraries as well). then, when you've added a bunch of Jenkinsfiles, you can mass import them into jenkins as long as you're using github or bitbucket for source control.
https://jenkins.io/blog/2017/02/03/declarative-pipeline-ga/ post is maybe a reasonable starting point for getting into declarative pipelines.