Treat a move to Minecraft 1.21.1 as a migration, not a file rename. At the September 2026 reference point used for this site, the public Tinkers' Construct project listing names 1.20.1 as its newest listed Minecraft line. That means a 1.20.1 jar should not be treated as a 1.21.1 build; wait for an explicitly targeted release and matching dependency set.
Do not assume nearby Minecraft versions are interchangeable
Minecraft mod binaries are tied to specific game and loader APIs. A build that works on 1.20.1 should not be treated as a 1.21.1 build unless the project explicitly targets that version.
Check the loader before the mod list
A profile can fail before individual mods are evaluated if the loader is wrong. Establish the supported loader branch first, then choose mod files that belong to it.
Review dependency availability
If a Tinkers build requires Mantle, the matching Mantle build must also exist for the same game and loader environment. The same rule applies to optional integrations you consider essential.
Protect your current world
Make a complete backup before migration. For major version changes, testing a copy is safer than converting the only version of an important world.
Plan for changed recipes and behavior
Even after a successful launch, major Minecraft or mod updates can change progression, recipes, defaults, or integrations. Re-check guide material after upgrading rather than assuming older instructions are exact.