So I did two tests - one just drilling to #30 and not increasing the dimple and one drilling and dimpling
With just drilling but leaving the dimple as is, the CS4-4 rivet head sits just slightly high. Just enough to catch a fingernail. With the other test, after dimpling to the larger size, I suddenly felt silly even asking this question because it's not like I'm working the metal back and forth. I'm just deforming the metal just a little bit more in the same direction it was going in the first place. So that turned out fine.
Van's support recommended using MK-319-BS rivets which would require drilling to 7/64" (or #34) and would work in the same dimple. I tested one and it would look better from an aesthetic point of view since that row of rivets will have a mix of pop rivets and AN426AD3 rivets. I would have to buy more MK-319-BS rivets so I thought if I'm doing that, why not just get 3/32" pop rivets like the Cherry "N" CCC-32 rivets? That way, I wouldn't have to re-drill the holes.
Cheers,