A 401(k) plan experienced a plan coverage change during 1998. In running the ADP test for 1998, the plan uses the prior year testing method. In determining the weighted average of the ADPs for the prior year subgroups, how do you treat employees who are mandatorily disaggreated? For example, one prior year subgroup has 200 NHCEs, of which 50 are union employees. Do you include all 200 participants in the prior year subgroup? If so, this would constitute a minor plan coverage change. Any thoughts? I haven't gotten through to IRS on this. Thanks.

union portion of the plan should be tested separately from the non union. (In other words, for all practical purposes, you have 2 plans) ACP test is deemed to pass (see 1.401(m)-1(a)(3)Thus no multiple use test for the union portion of the plan.
if ee shifts status, only include that portion of the year ee was union / non union.
see 1.410(B)-7©(4)

Based on your example, if I understand your message, last year you had 200 ees.
My prior years tests are as follows:
150 NHCE nonunion ADP and ACP average
compare to this years HCE nonunion ees

50 NHCE union ADP and ACP average copmare to this years HCE union ees.

at least that is the way I understand it.