That doesn't replace oil/gas boilers. You can have all of the solar and wind you want, and you'd still guzzle gas in the winter using oil boilers in most of the country.
The could use excess wind and solar electricity to power coal gasification plants to generate natural gas that they can then use in the winter, without having to retrofit any boilers. They can even make heating oil from coal:
https://en.m.wikipedia.org/wiki/Coal_gasification
They sure could, and may end up doing that but I'd bet it's going to be politically unpalatable.
Both retrofitting and coal gasification could probably be done in parallel.
It ain't gonna happen in three months, but if you have all of the solar and wind you want you can stockpile synthetic methane in the summer and burn it in the winter.
You need hydrogen to produce synthetic methane, and plants to produce it. The larges source of H2 globally is from you guessed it, natural gas. To get if from electrolysis, you need an electrolysis plant near your synthetic methane plant and both need to be near water because it's difficult and dangerous to store H2. All of that would take more than months. You could probably replace a large number of boilers in 3 months. Not enough to solve the problem, but enough to make a dent. Electric boilers, while less efficient than heat pumps, only cost around $700 each and are basically a 1 for 1 replacement for a gas boiler.