This is for Cisco but I would imagine it applies to others as well:
Whenever there is a change in the routing policy, the BGP session must
be reset (cleared) for the new policy to take effect and the routing
table to be reconfigured. Using a hard reset to clear a BGP session
causes cache invalidation and results in a negative impact on network
Soft reset is recommended because it allows routing table policies to
be reconfigured and activated without clearing the BGP session. Soft
reset is done on a per-neighbor basis. Soft resets can be inbound or
But the same document also says:
This feature provides an additional method for soft reset that allows
the dynamic exchange of route refresh requests and routing information
between BGP routers, and the subsequent re-advertisement of the
respective outbound routing table. Soft reset using the route refresh
capability does not require preconfiguration and consumes no
additional memory resources.
To use this new method, both BGP peers must support the soft route
refresh capability, which is advertised in the OPEN message sent when
a peer sends its routing table update.
Since you say you see no capabilities, there is no "route refresh" capability being advertised so there is no choice but to reset the BGP session in order to effect the change in routing policy.