Control Group to Multiple ER plan

LLP with 3 PAs (1 for each dr, LLP covered shared staff) broke up in February 2018.  One of the drs kept the LLP and his PA.  Some of the staff left, as did the other 2 Drs.  At this time we assume those two PAs are still in existence (they were on 12/31/18, PYE).  All are still participating ERs of the plan.

When would the plan be considered a multiple ER plan?  For 2018, the year the partnership broke up?  Or for 2019, the first full PY that they were not a controlled group of companies?

If the other 2 PAs (drs) cease to be participating ERS, when would that "kick out" the multiple ER "designation" - in the year it happened (like 2019 for example, if the plan is amended) or the year after the amendment?  See where I am going with these questions?

TIA!