This has not proven to be very historically true. The only subset of Trumps base amenable to Democrats are the populist leaning independents who want outsider candidates to upset the status quo. Joe Biden is not that.
I think Trump's base has a rasonable amount of people who have voted Democrat and would again for the right person. Blue collar workers who do not want subsidy from the government. These guys were a major part of the democratic base historically.
If Dem party goes with the sanders wing who would like a leftward lurch, these people will likely start identifying as reliable Republicans by default. It will be a calculated risk that could have long term results well past the Trump era.