This isn't as simple as corp vs. llc, the company in question is already incorporated. Investors have invested in the incorporated company. However, the founders have ownership through an LLC.
Then the issue isn't about the LLC but that only three out of the six original founders are active. I would try to find a way to reduce the 80% ownership of the LLC and give the difference to the active founders. Or to find a way to buy out the three non-active partners. If that's in fact the objective, then keeping the LLC might actually be quite useful. Keep in mind that when you start to make money, all proceeds distributed to the LLC is subject to self-employment tax, something that might not be optimal for the non-active founders. In short, find a knob and start turning. Good luck.