There is already a thread asking whether MagSafe1 chargers can be used interchangeably. However, having gone through all answers, and asked in the comments, I found that not a single person reported using a higher-wattage adapter on a lower-wattage MacBook successfully.
Ordinarily there should be no problem using a higher-wattage adapter, since the computer would only draw the amount of watts it needs (see the highest-rated answer there) -- but Apple thinks differently. They designed their higher-wattage adapters not only to supply more watts, but also to supply more volts to the computer. For example, the 45W Magsafe1 adapter supplies 14.5V, whereas the 85W Magsafe2 adapter supplies a whopping 20V (!). And in most cases, unless the port is specifically designed for the higher voltage, 3V over the spec is enough to fry the power port of the computer.
The accepted answer on that page quotes Apple as saying that we can use a higher wattage power adapter, but we cannot use one with less wattage without potential operating issues. However, this page has since been removed. I called Apple to ask about it, had to get transferred to a supervisor, and he hinted that Apple removed that statement because they did not wish to have liability to people whose power ports have burned up.
However, the one thing the supervisor did not answer is, how likely is it for a power port to burn up because a higher-watt charger burned up. I bought a couple of the 20V adapters without realizing they'd be at a different voltage, and I am afraid to hook them up to my 14.5V MacBook Air. However, the supervisor said that this is a question for the Engineering -- but Engineering support is not available for hardware which is no longer supported. Makes sense. But now to get the answer I have to ask the question here.
Has anyone heard of an 85W adapter successfully being used on a MacBook Air?