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In either case, the most practical way to apply exceptions to the default is via the use of the Algorithm Metadata Filter, to attach the standard extension to an SP's metadata to denote it as supporting one or the other algorithm. Essentially you're using the filter to specify a different default algorithm for that SP.
The documentation of the filter includes a couple of useful examples that provide suggested approaches for gradually adopting the use of GCM while maintaining the default in place. Similar approaches would apply in the event that the default were changed back but migration were to be attempted by applying GCM via filter to selected SPs.
In effect, while it may be eminently practical to use CBC as the default and move gradually, we felt that changing the default going forward was the only effective means of raising the visibility of the issue.