Well, Liquitex varnishes can be intermixed for a customizable sheen, when still wet, that is. That makes me guess that even when dry it would not be a big deal if you re-coat with a different varnish.
However, I have never re-varnished a painting with a different sheen coat myself, so I'm only writing my best guess.
I would give it a try. However, if you care a lot about this particular painting, maybe contact Liquitex directly - they have a contact form on their website - and see if their experts have different suggestions.
If you decide to re-varnish, me sure that your surface is clean from any dust, dirt, grease, or other deposit. You don't want to encapsulate those in between varnishes.