The reason to change oil (in a diesel) is that the additive package becomes depleted by neutralizing contaminants; the main contaminant is sulfur. LSD and ULSD should lower the depletion rate of the additive package, but I've not yet seen any reports about the degree of improvement. Synthetic oils don't really provide enough benefit to outweigh the costs (IMHO) until more is known about the effects of ULSD.
As for me, my motorhome's AMG engine gets RotellaT 15/W40 at 3K intervals (longer - to 4K if I'm on a long road trip). My Ford 6.9L NA gets the same treatment.
There should be no issues with switching to synthetic at any time. This idea may have come from the aviation side, where a new engine (or one with a rebuilt jug) should be run on non-detergent oil for a few hours so the rings will seat.
'94 Barth 28' Breakaway M/H ("StaRV II") diesel pusher: Spartan chassis, aluminum birdcage construction. Peninsular/AMG 6.5L TD (230HP), 18:1, Phazer, non-wastgated turbo, hi-pop injectors, 4L80E (Sun Coast TC & rebuild, M-H Pan), Dana 80 (M-H Cover), Fluidampr, EGT, trans temp, boost gage. Honda EV-4010 gaso genset, furnace, roof air, stove, microwave/convection, 2-dr. 3-way reefer. KVH R5SL Satellite. Cruises 2, sleeps 4, carries 6, and parties 8 (parties 12 - tested).
Stand-ins are an '02 Cadillac Escalade AWD 6.0L and an '06 Toyota Sienna Limited.