Varian, the Labour party conference attended by thirteen thousand set out the "red lines" for Brexit policy based on upholding the referendum result. However, since that conference, the trades unions have constantly been referring to their branches for feedback on any change those grassroots units feel is required to that policy.
In the above, a change in policy of the largest unions has come about by way of those branches, district and regional conferences. However, the policy adopted by the Labour party conference remains in being, and can only be changed by a recall conference.
That stated, I believe that should a general election be called a recall Labour party conference is under the revised rules obligatory to confirm all polices for the manifesto. Without doubt, it would be at that conference that a change to leaving the EU as policy would be adopted I feel.
The Brexit situation has changed dramatically since September, and keeping such a huge organisation in line with those changes has proved to be difficult, to say the least, especially when those changes in policy has to involve all at the grassroots sectors of the movement.