When Network Rail took over from Railtrack, Network Rail Ltd bought Railtrack PLC including all the assets and debts of that company. Note that Railtrack Group PLC (which owned Railtrack PLC) was not wound up by the courts, but instead changed name to RT Group PLC. It decided to sell off all its assets and went into voluntary administration. Railtrack PLC having gone into administration via a court order. But nearly all of the staff who actually maintained the infrastructure where employees of contractors.
Throughout the process, the railway infrastructure and maintenance continued as normal. Obviously some renewals and infrastructure improvement works/projects were affected.
When Great British Railways takes over, I would expect a seamless transfer if indeed that is what happens. But more likely, is that either a new holding company will be formed and then take control of Network Rail Limited (which in turn controls Network Rail Infrastructure Ltd via a holding company) or they will just change the company names.
Will there be a reduction of staff and therefore be redundancies? I think it’s far to early to tell. On the infrastructure (maintenance and capital) side, by that time, I think no. Because the reduction of staff will happen while Network Rail exists in its current form.