We just recently upgraded our rancher server from 1.6.12 to 1.6.28 too.
After upgrading, rancher gave us the option to upgrade the lb services too (a little upwards yellow arrow appeared near the lb service name in the load-balancers stack). As far as we witnessed, lb services were working out of the box after upgrading, with version 0.7.15. We then upgraded them to version 0.9.11 with no consequence.
On a side note, please take a look at the release notes for rancher server version 1.6.15 (NFS drivers changed).