The game is about 35Gb I believe now , switching patch methods does involve a new game download, if you have started this per the guide then it is best to let it complete.
Unspecified error as detailed at this guide link is an error that only occurs with the other (streaming) patch method, hence why it is a 100% fix to remove that error.
My recommendation is to leave the download running now until it completes, patching after this point will be a lot quicker
It Can be done directly, there is no need to have the fully game installed previously to swap patch methods , once it is fully downloaded the play button will be available
