Hi Sergey and welcome to Atlassian Community!
Currently, it's not possible to migrate from Classic to Next-Gen or vice versa, because next-gen is an independent type of project and nothing created for a classic project can be used on next-gen.
The best option, in this case, is to create a next-gen project and move all the tickets from a project to another.
Related to the roadmaps, we have a feature request suggesting the implementation of roadmaps on classic projects:
Please, click on vote and watch to receive updates about the feature.
Catch up with Atlassian Product Managers in our 2020 Demo Den round-up! From Advanced Roadmaps to Code in Jira to Next-Gen Workflows, check out the videos below to help up-level your work in the new ...
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