Fare absolutely DOES matter for upgrades on DL. However, it only matters within your upgrade window. There are many posts on the DL forum about this topic, but in essence, the DL upgrade process goes...

Anytime with V availability on Full fare Y for DM, PM, GM, FO
7 days DM - Amex Reserve, B,M, ... , L, U, T
5 days PM - Amex Reserve, B, M, ... , L, U, T
3 days GM - Amex Reserve, B, M, ... , L, U, T
1 day FO - Amex Reserve, B, M, ... , L, U, T
Less than 1 day MVPG/75K, MVP, ST E+, Medallion companions - Amex Reserve, B, M, ... , L, U, T

This means a MVP on a B fare would trump an MVPG75K on a T fare on DL because we are all within the same upgrade window.