Doubful that any downguaging will come without frequency growth if DL needs to pull the widebodies for use elsewhere.

Specific to UA, I think one can argue UA at JFK is irrelevant, and that won't change for a long time, regardless of DL/AA/B6 product--whatever schedule/equipment offered really just competes against/complements UA's offering to EWR for SFO/LAX premium customers at the moment.

It also strikes me that this inititiative is potentially aimed at competing better with B6. Offering a cometitive hard product with more limited seating and the ability to flex on frequency could significantly help the route economics. Even a small increase in paid J yields would help offset higher catering costs to compete with B6 as well (although I'm aware it comes at the expense of upgrade availability).