The trie format of substrate has changed recently in order to become more PoV-friendly. Once done a
state_version field has appeared in each chain’s runtime specification, where 0 is the old format, and 1 is the new format.
Migrating to the new trie can happen lazily, and technically, setting the version to 1 will put your trie in a hybrid mode where keys will be lazily migrated as they are read. This is okay, but it is a small overhead and it might take a long time until the state is fully migrated.
This is why we wrote a special pallet that can be configured to read and write all storage keys in the background, in a safe way, to facilitate the migration.
I wrote a guide about this a long time ago, which some of you might have already seen, but I want to re-share it again: Substrate State Trie Migration Guide - HackMD.
I am aware that Parity is in the process of migrating our testnets using the mentioned tool and method, and I will ask them to share their outcome here once done. Until then, I suggest being extra cautious when it comes to performing the migration.
And we can use this thread to share our experience performing the migration and help with troubleshooting.