It would be super cool if we have the option to change the item needed to upgrade racks. Currently you can only upgrade with the default items (stone, iron, etc) by left click. My suggestion is that you can have a config option for each tier and the corresponding item needed for upgradin them. This will allow modpack creators to set the items we want for custom progression.

Example:
rackstoneupgrade: minecraft:prismarine
rackironupgrade: minecraft:nether_star

Thank you!