I manage several sites, some of which have hundreds of pages. I have always been a fan of naming pages with your primary key phrase e.g. /primary-key-phrase.html - this seems to be quite productive. The only problem I am finding is that when I want to re-tune pages for new keywords/phrases it, you obviously have your old ones in the name of the webpage.... It is fine to tune a page for a sceondary key phrase. Is the answer to create a new page or do you think a keyphrase rich named page is not that imporatnt. A new page is unlikely to have the authority as that of the old one. ???
The search engines can associate two phrases so as long as the keywords are broadly similar it shouldn't make much difference. If you can confidently create a strong new page to target the new term and make it a lot different to the old page then do this instead.