But not in v29.0 once it's released
If it won't be capable of converting it as needed, that's a surprise.
Update; there was an update in the proposed timeline edited 3 weeks ago that I haven't checked:
You wont be able to load your legacy wallet in v30.0 instead of v29.0. In that release, Bitcoin Core will provide instructions on how to migrate your old wallet.
But if you want to do it now, use "
File->Migrate Wallet".
Though I'm wondering more about potential "silent" problems.
In theory, one would expect it to explicitly alert if something's not compatible, leave the data be, and exit gracefully.
Core is typically pretty elaborate in data compatibility checking, and logging / alerting.
Those are more typical to updated config, network, etc. which depends if you're using such options.
Like for example, v28.0 and v28.1's update in bind settings which could break some setups.
But for the Blockchain and Chainstate compatibility, it's should be explicitly noted in the release notes.