I'd say that your idea is a good one strategically but it's not going to be quick and easy.
If it were me, I'd start not by revising anything on-page, but by working on building links back to your home page from content relating closely to "merchant services" and with anchor text that's somewhat thematically relevant to that. Once you've build up your domain and page authority to be somewhat equal to other sites that are ranking for your term, then you might revise your home page meta tags and content to take advantage of the new strength. The reason I say I would go in that direction is because your home page likely isn't currently strong enough to rank for the new keyword (just by virtue of on-page changes) and it may not be strong enough to pass the strength that the internal "offshore merchant accounts" landing page would need to take up the slack created by the home page not ranking for that term any more.
The better the quality and relevance of the links to your homepage, the quicker the transition will take place in the search results once you make your on-page changes. Right now, your link profile is not terribly strong.
And when you're ready to make your on-page changes:
- Go through your site and make sure all links with anchor text of "offshore merchant accounts" are revised to point to the internal page that deals with your offshore merchant account service.
- Make sure that links pointing to your home page carry the anchor text "merchant services" or variations of that.
- Anchor text from your external links going to your homepage looks generally OK but where possible, you should work on making them variations and synonyms of "merchant services", rather than "offshore merchant accounts"