Hello,
We did upgrade from 1.5 to 1.6.0p27 with customized tag group agent in place and had no issues with that. Did you tried to first apply the upgrade and later fix the tag group?
As far as I remember the process was that as soon as all legacy tags removed from the tag group it switches automatically to the built-in in 1.6.
Deleting the tag group agent probably cause the stack trace.
and tag_groups["agent"] == "no-agent" \
KeyError: 'agent'
regards
Michael