Of course it's not going to happen. How complicated the regauging would be makes a difference and is more important that how much money it costs. How it would be done it if were done makes a difference to the inconvenience of the conversion. I know dual gauge track is possible, but it needs narrow footed rails, and also places a speed restriction on broad gauge rolling stock. Would the need for narrow footed rails and that speed restriction complicate regauging?
If the system was to be re-gauged the preparation would take years to complete, where the existing sleepers would be changed out and replaced with new full size gauge convertible concrete sleepers (as per Adelaide)
Major rail mounting conversion work would be needed In the likes of the City Loop
BG Turnout points would probable be fully replaced by new SG points (all delivered and set up line side)
The major train maintenance and stabling depots would probably be temporarily expanded and dual gauged to permit the re-gauging of rolling stock (thousands of new or refurbished SG wheel sets would need to be on standby)
During a major system wide shutdown during Christmas, New Year, January holiday period.
Trains would be re-gauged,
The BG rail would be un clipped and moved over the SG position and clipped back.
Loading gauge clearances through platform pits and past bridges would be adjusted.
BG turnout points would be change out and replaced with SG one's.
Testing and driver training would be conducted before everything gets back to normal.