Due to the joys of SACK Panic, we have begun migrating out 1.1.8 older routers to 1.2.1 finally.
We started with one of our EDGE routers, which BGP's to an upstream 3rd party for IP Transit, and then speaks to two of our core routers by sending them a default route. The Core <-> EDGE is handled via OSPF for finding each other, but BGP exchange for distribution to rest of network. Core Route Reflect to a redundant EDGE router to another router. Most of this is I presume not relevant to this issue.
On upgrade, the router had assumed all correct interface IP's, and all OSPF routing was working fine.
BGP simply was not started. It showed no current config, no neighbours, no AS. This is despite the 'show config' showing our previous fully functional BGP config.
By deleting and re-adding the same lines in the protocol -> bgp portion of the config, FRR BGP daemon slowly came online. It was a little bit of trying each line one by one to see which parts would make FRR respond. It appeared to take the neighbour config in part, but would not set the nexthop-self or remote-as parameters until they were deleted and then re-added in their seemingly identical format.