I have established a few of these "Solo" plans. Up till now the possibility of rank and file EEs becoming eligible has never been an issue. It was spelled out to the sponsor that if an employee was hired that they would indeed be eligible to participate in the plan IF they met the requirements in the document. Typically the sponsor would make the eligibility requirements pretty lax... Here is the question:

If you have set up a plan to have immediate 100% vesting and 0 years of service requirement... can it be amended to a TH vesting (2/20) and one year/1000 hour service when you see on the horizon that there are going to be other EEs who may be eligible to participate? is that discrimination?

You can't discriminate against a potential participant. It is okay to amend the plan before their date of plan entry as you suggest.

This is also a lesson that the plan should initially be set up with the anticipation of future employees so you don't have to worry about such issues.

If I was an IRS auditor, I would seek these Uni-K, Solo 401(k) or whatever you call them plans out in abundance in a few years. A qualified plan without a qualified person to monitor their existence means a host of problems will arise.

I agree...