One thing to note, most P&S agreements have timelines for completing obligations by both parties. The P& S will need to be modified to accommodate the fact that the boat is hauled and more than likely blocked in by other boats. It requires flexibility.
For example it is really neither parties fault if the marina is frozen or the yard can't move 7 boats to accommodate your seatrial. Both parties are going to need to be flexible and anticipate date extensions. Both parties are going to need to be flexible in that the timing for the buyer, seller, broker, boat surveyor, engine surveyor, and yard may not easily coincide. Again, P& S dates may need to be amended.
The seller or broker should be the one working with the yard directly since he holds the storage contract with them. You should only need to worry about working with your subs.
Regarding P& S changes, make sure you not only sign your copy, but get a signed copy back acknowledging the change in the terms of the contract. You always want a copy with the most recent changes and BOTH parties signatures.