I will reply to only a single part of your question since I don't have the SEO experience the others seem to have.
Are there any other aspects to consider?
A few years ago I built a website for a local BBQ restaurant. After about a year they decided to do some shuffling and changed the BBQ restaurant to a Bar & Grill, bought a building in another city and made it a catering hall, and opened a food truck in another area. Since each part of his business was in different areas and was slightly different, but still selling similar products/services, this is what I did.
I modified the original BBQ restaurant content and added a separate discrete site navigation to each page that would take you to the respective page of the other part of the business. The idea behind this was that the Bar & Grill held the original design to not confuse customers due to the change. The Catering site was much cleaner and focused on formal events, but still had a couple similar design qualities as the Bar & Grill (i.e. - similar background and navigation). The food truck site was had a very similar design to the Bar & Grill but was better able to work on mobile devices.
We kept the original domain pointing to the Bar & Grill website since that is what they were known for and how we expected customers to search for them. We expect this because of our naming convention. For example, Robert's Bar & Grill, Robert's BBQ Mobile, and Robert's BBQ Catering & Banquet Hall. The legal name, using the fake example name, was Robert's BBQ but each of the others was a dba (doing business as) to try and separate each but keep them together.
I hope this helps and doesn't confuse!