How can I move from a financial content career into a writing/editing (sub editing or copy editing) field?

It helps to be familair with the various stylebooks used for professional writing and editing, like the Chicago Manual of Style, and New Hart's Rules: The Oxford Guide to Style, as not all clients use the same style (US vs British spelling, grammar and punctuation). Also you should know that for academic writing, there are specific formats that must be observed (like APA format), and various journals have their own specific style which must be followed for articles being submitted for publiction. While spelling and grammar check are great, they usually will not weed out words like there vs their, so you really need to be on top of your game. 

editing is basically styling of the content based on the client requirement.It requires mastery in grammatical errors, spell checks and identification of errors in punctuation.If these areas are strong one can move into the field of copy editing