How efficient is the changeout? What is the equipment required? How much skill is required of the maintenance crew?
One of the reasons for the incredible logistics run by the U.S. in WWII was the standardization and modularization of the machinery.
Never done a retube on a M777; after my time in the Army. On a M198 towed 155mm howitzer, direct support maintenance could do the job in a day or 2, if authorized and if parts are on hand. Still is typically a depot level job. Again a day or two. Normally, we would get a howitzer to replace one going to depot maintenance so that we kept our combat readiness. Recoil mechanism on the 777 looks more complicated than M198, so retube might take a little longer. Still days, not weeks, assuming barrels and mechanics are available.