Some quick history of the revolution up till now: Until the late 1970s, pretty much all voting happened on Election Day, with the main exceptions being military personnel, who were allowed to cast absentee ballots. But in 1978, California became the first state to let any voter request a mail-in ballot for any reason. And many other states followed suit.
Flash forward to today, and five states have switched to full vote-by-mail. Oregon became the first in 2000, following a 1998 statewide referendum. Washington (2011) and Colorado (2013) joined more recently, followed by Utah and Hawaii. Other states, especially in the South, have expanded in-person early voting, with Texas starting the trend back in 1987. But there are still a number of states that will need to play catch-up if the November election relies heavily on remote voting. In total, 10 states lack any type of early voting option and 17 states lack no-excuse absentee balloting, meaning they have limited experience handling mailed ballots.
The potential effects on 2020 are hard to predict from past elections, though. On balance, early voting appears to have raised turnout a little, at least in some places — but only by a few percentage points, and with no clear advantage to either party. But results vary, both by which group of voters you’re looking at and by the type of election. Turnout, for instance, has increased most among older people who tend to vote at the highest rates already. And turnout has generally gone up most in lower-profile elections, like primaries and off-year contests. But the effects on general elections are more limited, perhaps because more people already tend to vote in them.