presumably they might've stripped some features from the latest version of origin in anticipation of the migration.
just download an older origin version manually and overwrite your origin install with the files and stop the update happening
how is the ea app still incomplete and broken is just terrible
and it doesn't even work on 32 bit computers (not that games these days will probably run on them, but no way does a 'lightweight' launcher need to be 64bit)
so far the ways of blocking the update prompt are
1. blocking origin from checking for updates and thus knowing it needs to be migrated (only works if you can restore your origin program and app data folders to before the popup showing, OR the settings xml/whatever being restored. THIS WILL NOT WORK if the popup is already showing.)
2. modifly some files (potentially breaking features or whatever)
3. changing xml settings in any origin version (latest version seems to be lacking in some features?)
