Thus far, UA has said they are not cancelling longhaul widebody routes, so presumably no. And to the extent widebodies are used on domestic or short-haul international routes, those frequencies are presumably the least likely to be cancelled since it would displace many more customers than cancelling a regional jet, for the same amount of ATC capacity freed up.

There are some narrowbody cancels, so I guess maybe combining a few RJ flights into one NB operation could happen if needed, but that's a lot of extra complexity to find time to manage on top of all of the operational disruptions created by the cancels in the first place. If this drags on into the holidays you might see some of that though.