No Short-cuts Available
In a well-designed research project dairy calves were weaned either abruptly (7 days at half milk ration) or gradually (7 days at 60% followed by 7 more days at 30%).
Using several measures they estimated rumen development and ability to digest solid feeds.
The calves with the longer weaning period had superior rumen development at the end of the weaning period.
No surprise. The calves need to be eating a measurable amount of solid feed for at least three weeks before the end of milk feeding in order to have a level of rumen function that allows for replacing the energy from milk with that from solid feed. No short-cuts available. Biology always wins, no cheating.
Another research group suggests that a calf starter grain intake of at least 4.5 pounds per day is desirable for moving from a milk+grain ration to a 100% grain ration if consistent gain is desired (assumes moderate environmental temperature).
Reference: Klopp, R. N. and Others, "Effects of feeding different amounts of milk replacer on growth performance and nutrient digestibility in Holstein calves to 2 months of age using different weaning strategies." Journal of Dairy Science 102:11040-11050 December 2019.