The website is CMS based and was initially launched on topic A which has niche appeal. However the site has grown over the years to also encompass topics B and C which are much bigger and more prominent on the internet today than topic A ever was. For as long as I can remember topic B has attracted 75% of all the organic traffic the site gets from google.
Starting a couple of years ago I changed all the meta data of the site to better reflect my new desired focus on topic C and stopped looking for links to it from sites on topics A or B. I also focused heavily on creating content on topic C and relevant back links from sites related to it with appropriate anchor text.
1 year into this process I decided it was appropriate to rename the domain name from which the web site is served from a name including keywords X and Y which related predominantly to topic B to a name including (the same) keywords X, Y and (a new keyword) Z which changed it's relevance very heavily towards topic C. I also 301 redirected all the old URLs from the old domain to the new, and informed google of the change as discussed in the following thread:
Another 8 months later seeing maybe only a 10% change in favor of topic B. I moved all the content on that topic to a whole new web site and domain name (using the same old keywords X and Y but a different TLD extension), 301 redirecting all the old URLs on topic B from from web site 1 to web site 2. Website's 1 and 2 are linked to each other however and hosted on the same host and IP address.
Regarding back links I 301 redirected all those related to topic B to web site 2 and focused on legitimately generating as many as possible new links towards web site 1. I also did my best to have as much anchor text updated around the change as possible.
However 6 months on again, organic referrals are showing maybe another 15% to 20% increase in favor of web site 2 only and no change at all to web site 1 which is still attracting traffic mostly related to topic B.
Competitiveness is an issue. Topic B is not in a commercial sector while topic C is however web site 1 is definetely sizable and relevant enough to rank at least partially for topic C...