My prediction-
Single licence mech guys with 767 or 744 will be first in the firing line. The first on first off/seniority thing will only come into it if it comes down to two or more guys with the same licence.
I'm not saying it's fair but do you really believe they'll send someone with a 330 or 738 to the hangars or tell a newly appointed DMM to put his overalls on for that matter.
Like I said, just my prediction...
-JA1