> how can a senior software engineer shift his area of expertise without going through being a junior again

Find something related act as a senior eng on.

That could be an internal project, an open source project, or a side project. Without more context, I'd suggest finding an open source project that interests you. If you can use it at your work, all the better. Spend some time improving that, learning the community, bug fixes, working on docs/workflow, etc. You know, senior engineer kinda things.

Then, next time you are applying, you can point to that project.