Update: This sort of thing is now impossible...
Please note: there's actually a fairly significant bug with this right now, that results in one of each of the potential behaviors happening depending on whether you're the asker or answerer (one gets source-site rep stripped, the other keeps it). So this is sort of the worst of both worlds. The quick and sane fix is probably to just return to the previous behavior: migrated posts no longer count toward reputation on the origin site, but count on the destination.
That said, there's a reason migrating very old, popular questions is discouraged: that answer is now by far the highest-voted question and answer on DBA.SE. For reference, the highest-voted question and answer actually written on DBA.SE scored less than half of what this SO refugee got... This doesn't reflect the value placed on it by DBA users in any way, shape, or form; meanwhile, the post and score are disconnected from the site that actually did value it.
Frankly, I can't think of any good reason why a popular, on-topic, answered question should be moved from one site to another years after being asked. I've reversed this particular migration.
Resetting all post scores on migration would probably work now without being too unfair to folks who've legitimately contributed something to one site just because it gets bumped to another. This would give each site's local community a chance to rate and rank it according to their own standards. In the past, this would have seemed horribly unfair (in particular for older questions like this one that were perfectly on-topic for their original site and legitimately earned that rep). I'm ok with it working this way, but again... Most of these posts should probably never be migrated anywhere. Resetting votes and reputation on a day-old post isn't a big deal.
Note that "network reputation" isn't really a thing. Your combined cross-site rep makes your Area 51 commitments count for a bit more, but other than that it doesn't really give you any special privileges anywhere.
Right now, there's a bug that needs to be fixed. Beyond that, I think the "solution" to this is probably just to discourage or block these migrations entirely except in very, very exceptional circumstances.