Save a baseline
Back up the instance, world saves and configs before changing the library. Record the Minecraft and loader versions plus the current Placebo and dependent mod releases. This baseline makes rollback possible even if a launcher silently changes several dependencies.
Test the intended combination
Create a copy of the pack and check the new file's Minecraft tags, loader and dependency ranges. Read available release notes, then change only the components needed for compatibility. Launch the copy and test the gameplay systems you use rather than stopping at the title screen.
Promote or roll back
When the test copy works, apply the same version set to the live instance. For multiplayer, coordinate the tested pack with the administrator. If failures appear, restore the baseline and compare logs; do not repeatedly open a valuable world with an uncertain mod combination.