The EAS says something about the work, but as usual not a lot:
12 DAY BLOCKADE. GWR▸ SERVICES BETWEEN
LONDON AND THE WEST OF ENGLAND TO RUN
VIA WESTBURY AVOIDERS. GWR AND SWR»
SERVICES TO TERMINATE AT / START FROM
WARMINSTER AND TROWBRIDGE. GWR
SERVICES BETWEEN BRISTOL AND WEYMOUTH
TO REVERSE AT HEWYWOOD ROAD JCN▸ .
FREIGHT SERVICES TO / FROM SOMERSET
QUARRIES TO RUN VIA WESTBURY AVOIDER. NO
ACCESS TO / FROM ANY SIDINGS / STABLING /
LOCO FACILITIES AT WESTBURY. NO ACCESS TO
/ FROM LDC AT WESTBURY EXCEPT FOR TRAINS
REQUIRED FOR THIS POSSESSION.
WORK = WESTBURY NORTH S+C RENEWAL AND
IEP▸ SIGNALLING WORKS WESTBURY STATION.
SPECIAL WORKING INSTRUCTIONS REQUIRED
FOR REVERSALS AT WARMINSTER AND
HEYWOOD ROAD JCN (gpl 732 SIGNAL).
"Renewal" doesn't sound like a change at all, but I think it's used to cover any replacement, not just like for like. But I don't think it extends to new tracks or platforms.
And as to "IEP signalling works" ... ?
Incidentally, that 12-day blockade runs 23/12/18 to 4/1/19, not just to the 1st. (EAS version from March.)