I thought I'd look at the Earl Weaver Clause in the official rule book. It's some windy baseball lore, for a day with no baseball.
Rule 5.11 (a) (2) reads as follows:
The Designated Hitter named in the starting lineup must come to bat at least one time, unless the opposing Club changes pitchers.
If you think about it - why would this clause even be necessary? Was there a rash of managers changing their minds about who their DH was going to be after they'd handed in the lineup card?
No.