Comment 4 for bug 1553287

Please note that truncating and repopulating metabib.metarecord will destroy any existing metarecord-level holds, along with historical hold data. That's one reason we don't remove old metarecords that have no more constituents, and in some cases avoid removing out-of-date mappings. It's also why we don't recommend wholesale MR remapping in general.

If your site doesn't use M-type holds at all, then it's perfectly safe.