I think "entire" is too strong a word.
Yes, you need charging, maintenance, supply chain, etc. but in very limited quantities at first. Usually, pilots are started with a limited number vehicles, staff, routes, and infrastructure. Similarly old vehicles are often phased out with as the depots, maintenance facilities, etc. are converted to support the new vehicles.
This is not only economically and environmentally efficient, but also operationally efficient. If you simply switch from one technology to another over a short period of time, you're opening yourself to minor issues causing major havoc.
The government doesn't guarantee the pension if the fund fails, they manage it so it doesn't fail.
EDIT TL;DR That means they're not financially liable to top up the pension, it means they get to decide how the pension returns to a healthy state, potentially by rewriting the contribution rules.
Most of the unions assume this means the next step is the government will stop funding their portion of the contributions instead of sharing the savings with employees. The step after that is if (really, when) the fund becomes unhealthy, the government gets to unilaterally decide how to fix the fund. Sure, they could top up the fund completely out of public revenues, but they could require employees to help top up the fund.
The reason why the unions think this will happen is because it has happened before. Any employee under the PSPP who's been employed for greater than ~15 years (I can't remember the cut off date) makes 35% of the contributions to their pension, with the government making the other 65%. Anyone who's been with the public sector under that cut-off pays 50%. That's because we started this same cycle back in the early 2000's with the government taking the surplus, but not putting the money back when investment returns were low. AFAIK, a similar cycle has happened at least once before that.