Scary can of worms to open. The same logic can be applied to wfh (your commute got shorter so you get less money or you live in a location with lower col so you get less money).Can a company just move offices without compensating their employees for relocation costs?
Often moving an office means you lose ~10% of your staff. That's not always a bad thing as they were likely not key employees. Where a company I know moved substantially (tens of km's but a far different commute) they offered nothing. Employees were free to leave. That worked. Almost 100% went.
As for MP, the job is where the office is now. Construction jobs often move offices to follow the work. I would suspect they include it in their contracts.