how much efforts we need to migrate all of our Phamton-pdf based reports to the Chrome-pdf version
It depends.... Chrome produces different sizes of elements so the output pdf is usually scaled compared to phantomjs.
if you have any Chrome-pdf examples in your playground so we can have a little taste of the new recipe
You can take a look for example on this one.
https://playground.jsreport.net/studio/workspace/SyUrRILTg/12421
if we can keep both Phamton-pdf and Chrome-pdf reports running simultaneously during transition
You just install additional extension jsreport-phantom-pdf and you can run both phantom or chrome based recipes in parallel.
So if you are happy with the current templates/outputs. Keep using phantom.
If you are interested in using more modern html/css/js, start creating the new templates with chrome recipe.