That is a very large increment of Jenkins versions, spanning multiple significant changes in the Jenkins project. Information about those significant changes are in the LTS changelog and in the upgrade guides. Read the changelog and the upgrade guide very carefully.
Your organization has chosen to not upgrade Jenkins for 3 years. That has created some “upgrade debt” that you’ll now need to repay. @mwp565733 shared experience with that type of upgrade in a blog post. It is worthwhile reading as you consider your upgrade path.