In vanilla Minecraft, upgrading a Diamond tool to Netherite adds 470 to both its current and maximum durability (from 1561 to 2031 as it uses Damage internally instead of a Health-like). This should be preserved such that when you upgrade a ruined Diamond tool to Netherite, it revives the tool, leaving a damaged Netherite tool in the output.

The same should apply for Diamond -> Netherite armor and (if possible, in general) any smithing recipes which boost the max durability of the tool.