After migration, it is impossible to update/delete triples from a 4.7.x repository that used language-tagged strings created in 4.6.x. Moreover, any SPARQL Update request that attempts to delete an affected triple, but which may attempt to update/delete other triples for the same resource, will fail.
(I'm assuming all versions of 4.6 and 4.7 are an issue here, but I've only confirmed 4.6.1->4.7.0 and 4.6.1->4.7.4.)
Two different behaviors have been observed depending on whether you are trying to delete one of these triples from 4.7.0 versus 4.7.4.
4.7.0: PATCH response returns "400 Bad Request" and the response body will include a message like "Property 'ns001:prefLabel': Unable to remove value 'Hello^^http:...."
4.7.4: PATCH response returns "204 Content Updated." But the triple remains.
This gist should reproduce the issue sufficiently: https://gist.github.com/kefo/3272ec4fa07da7ab9855db843922b295
it is possible that this particular bug is related to https://jira.duraspace.org/browse/FCREPO-2383. The error message is similar and
FCREPO-2383 was fixed in 4.7.4 and the response for 4.7.4 here is different than 4.7.0.