If you are revamping the site, it is a great time to find out what you can do to improve your rankings rather than worrying about losing them.
So, I would have the current SEO review the site and report back on what can be done to improve. Then I would make the same request of an SEO consultant. Once I have their recommendations I would compare, discuss with them, do research and make a decision about what to do.
Paying the consultant is not a waste of money. The price should be small compared to your potential sales increase going forward. It is also education for you.
This process might increase your understanding of your website, understand what the improvements can be and feel more confident about moving your rankings up rather than worry about losing them.