These days I am managing a blog whose niche I have to change. Previously it was optimized for a particular set of keywords in one marketplace but now I am changing it to another more focused one. There are a lot of difficulties coming along the way as well as sudden decline in traffic for sometime. SEO Professionals who have done this before, their input and suggestions would be highly invaluable to me.
Even I had done the same thing earlier because I had multiple keyword based domains. After Google EMD update, such websites became useless so I combined all websites together and it worked for me. Now getting around 2500-3000 daily visitors through that site.
If you are changing your niche then I would suggest that use new domain name according to your niche....
I can't speak for the technicalities of the SEO. But if you are concerned about the fall-off of traffic, then start getting engaged in the social conversation immediately. Create an RSS list of the big hitters in your niche and regularly start jumping in on their posts with a link back to your site. It'll do some short-term and long-term good while you're busy re-engineering the site.
Why don't you want to make a new website and start optimizing it? Or your old website has already had a trust?
You can also try working with themes that are closely related. I.e. broad digital marketing Specific social media marketing.
Maybe a good idea would be to create an additional section on the blog. You don't have to do drastic changes that could potentially harm you.
Yes the blog is old and has a considerable trust and backlinks. Plus its a project so I can't choose.