It may well drop a position or two it may go up a position or two.
Changing the theme will have little effect if any at all. The important thing is your KW density and the fact you have it in the right area to be ranking 7 for your chosen KW.
By all means look to improve your content but take some precautions first.
Interrogate the backlink patterns of the sites above you. Try and establish what it is they are doing what beats you.
Make a note of your current kw density and try and match it when you rewrite the content.
Look at the sites above you KW density, you may learn something from that.
If you do change and a drop occurs do not panic, sit tight for a two,three weeks and see where the site settles. G will re-evaluate it and may mark it down some spots before it marks it up