I'm not sure there is such a thing as a domestic hold file anymore. Any new hires would be straight to the 777. There may be some internal shuffling of numbers but no talk of externals gaining a direct entry position on the 737/Emb.
Having said that, that may change if there was a large push for more crew, (unlikely scenario) as, IMHO the company would not be in a position, logistically, to upgrade large numbers of cruisers and fill the slots they leave simultaneously.
One thing that is clear though is that F/O numbers are quite short at the moment and have been for some time. Whether that means too much ad-hoc leave was taken over the last few months or resource planing are just refusing to concede remains to be seen.
To those that get the call, nice one and well done for hanging in there.