"INVOL" is put as the fare basis to indicate, obviously, an involuntary reissue (meaning no add-collect). The operating carrier will be paid based on ticketing endorsement/proration rules. In most cases, like with a US-UA schedule change, it's just the original face value which still should be detailed in the fare calc line (e.g. "LGA UA X/CLT UA MEM 202.00QR0NX ..."). In some cases, which airlines try to avoid, it's a percentage of the point-to-point Y fare (no, not the coded Y, the REAL Y!).

My guess is that, in the event of a schedule change, you will only see it when there are codeshare flights affected by the sched change (e-ticket revalidations generally don't work on codeshare coupons). Reissuing a ticket costs money and is not necessary when it's all coded and flown by one company.

Just be glad that the reissue is done in advance. When I worked at JFK, we always had to do the reissue on the day of travel which made some mornings hell.