I can't help but comment: THERE IS NO SUCH THING AS A SOLO 401(K) PLAN! There is just plan documents that are crippled and probably screwing up clients all the time (we know that's true: they often end up on our doorstep and we have to "fix" things). If he never hires a real employee, his existing plan document can do all you need to. If the plan was well drafted, there should be a provision that says HCEs don't need to get the Safe Harbor allocation (but you CAN give them a PS allocation if the client desires). You can amend the plan to eliminate the safe harbor provision, but it likely doesn't matter. Hopefully, as noted by another, there is the standard 1 year/ age 21/ semi-annual entry date provision, which a lot of so called "solo 401(k) plans" have hard wired as immediate entry; that's a problem if, g-d forbid, he hires someone for even one day. They also tend to be hard wired for immediate 100% vesting; also should be avoided just in case they ever hire someone. Again, the "solo 401(k)" document is just a crippled document with bad provisions.
Bottom line: if your plan document is a good one, then just modify anything you really don't want and keep it. As far as moving it to a "new vendor", if you are just talking about the investment, a non-participant directed, pooled plan would make the most sense for this situation and those changes can be made to the plan document and you can invest the funds with anyone you want.