rikkiphillipson

Your part time employees would become eligible to defer if they worked over 500 hours in 2 consecutive years even if they don't meet the 12 month 1000 hour eligibility requirement. There are a lot of regulations to navigate to maintain compliance and plan qualification which is why as mentioned above you should have a TPA to guide you through your changes. Penalties can be very costly, and plans can be disqualified if found noncompliant. As far as your plan goes, you shouldn't have to "convert" anything. You may just need to amend your plan to meet your needs as they change.

MDO and associated match are deposited as QNEC and always 100% vested.

If they filed an extension on their business taxes it should automatically extend their 5500 deadline as well. https://www.napa-net.org/news-info/daily-news/case-week-form-5500-and-automatic-filing-extension

The match formula cannot be based on more than 6% of deferred compensation. The match cannot exceed 4% of deferred compensation in total. So, a discretionary 50% match on the first 6% of deferred compensation (3% total) would be exempt from the ACP test, while 25% match on the first 10% of deferred compensation (2.5% total) would not.