My attempt was a bit more crude:

diff --git a/nest/config.Y b/nest/config.Y
index aef5ed46..829bf96c 100644
--- a/nest/config.Y
+++ b/nest/config.Y
@@ -64,7 +64,7 @@ proto_postconfig(void)
 
 CF_DECLS
 
-CF_KEYWORDS(ROUTER, ID, PROTOCOL, TEMPLATE, PREFERENCE, DISABLED, DEBUG, ALL, OFF, DIRECT)
+CF_KEYWORDS(ROUTER, ID, PROTOCOL, TEMPLATE, PREFERENCE, DISABLED, KEEP_STATE, DEBUG, ALL, OFF, DIRECT)
 CF_KEYWORDS(INTERFACE, IMPORT, EXPORT, FILTER, NONE, VRF, TABLE, STATES, ROUTES, FILTERS)
 CF_KEYWORDS(IPV4, IPV6, VPN4, VPN6, ROA4, ROA6, FLOW4, FLOW6, SADR, MPLS)
 CF_KEYWORDS(RECEIVE, LIMIT, ACTION, WARN, BLOCK, RESTART, DISABLE, KEEP, FILTERED)
@@ -205,6 +205,7 @@ proto_name:
 proto_item:
    /* EMPTY */
  | DISABLED bool { this_proto->disabled = $2; }
+ | KEEP_STATE bool { this_proto->keep_state = $2 }
  | DEBUG debug_mask { this_proto->debug = $2; }
  | MRTDUMP mrtdump_mask { this_proto->mrtdump = $2; }
  | ROUTER ID idval { this_proto->router_id = $3; }
diff --git a/nest/proto.c b/nest/proto.c
index d4a333d0..397d6fb2 100644
--- a/nest/proto.c
+++ b/nest/proto.c
@@ -984,6 +984,8 @@ protos_commit(struct config *new, struct config *old, int force_reconfig, int ty
     if (! force_reconfig && proto_reconfigure(p, oc, nc, type))
       continue;
 
+    nc->disabled = nc->keep_state ? p->disabled : nc->disabled;
+
     /* Unsuccessful, we will restart it */
     if (!p->disabled && !nc->disabled)
       log(L_INFO "Restarting protocol %s", p->name);
diff --git a/nest/protocol.h b/nest/protocol.h
index 6c04071b..d984b4c2 100644
--- a/nest/protocol.h
+++ b/nest/protocol.h
@@ -118,6 +118,7 @@ struct proto_config {
   int class;                /* SYM_PROTO or SYM_TEMPLATE */
   u8 net_type;                /* Protocol network type (NET_*), 0 for undefined */
   u8 disabled;                /* Protocol enabled/disabled by default */
+  u8 keep_state;            /* Keep current enabled/disabled state during reconfiguration */
   u32 debug, mrtdump;            /* Debugging bitfields, both use D_* constants */
   u32 router_id;            /* Protocol specific router ID */