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Crossing vertically up-P from a hornfels to a regional facies does
not represent a change in grade, so nothing dramatic should happen.
An ignomineous petrologist
The change from amphibolite facies to eclogite facies is also not a change in grade, but something does happen. I hope you didn't mean crossing from contact to regional faies, because you might get stoned to death as things stand right now.
Perhaps one way of keeping the distinction between different P-T gradients alive is to use a fourth facies series term as in Blatt & Tracy (I haven't seen this in any other book so far). I don't have hang-ups about "hornfels" or, alternatively, maybe "very-low-pressure/high-temperature".
A metaphoric petrologist