The market has a financial tool for this, called "Solar for coal" swaps, where the existing debt is refinanced into new debt (at competitive financing rates) issued for building utility scale solar generators. Agree that the US gov or the central bank should step in otherwise to subsidize the closure of these plants. It's going to happen one way or another.

Not needed. This is what bankruptcy is for. Just don’t pay back the debt.

If it’s held by investors, well, all investments can go down in value. If it’s held by companies that build carbon emitting plants, even better.

Municipalities and non profit co ops will not voluntarily pursue bankruptcy to re-negotiate or default on these debts (which will also impact their credit rating in the debt market for future issuances). They'll have to be forced into it if outside support isn't provided.