It has nothing to do with any encryption. The encryption of the wallet file has not changed... it is simply the "seed_version" value being stored in the wallet file...
Electrum uses the "seed_version" value as a "wallet version" value and it is incremented whenever modifications are made that alters the wallet file format in some manner. This is to prevent new additions and/or modifications to the wallet file structure causing unexpected errors should an "upgraded" wallet be attempted to be opened in an older version of Electrum that is not expecting them.
So, when wallet file format changes are made, the "version" is incremented... and the wallet is upgraded the first time the wallet file is opened using a newer version.
At some point between 4.0.5 and 4.0.9, the version was incremented from 32 to 33... hence why, after opening the wallet file with 4.0.9, the wallet file can no longer be opened in 4.0.5.
In the future, should you wish to maintain the ability to "rollback" to the previous version in case an update causes issues... you should backup your wallet file before you update Electrum. This will mean you have a copy of the wallet file with the "earlier" version.
NOTE: not all updates actually modify the version... the wallet version was actually last updated in version 4.0.
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