My cover band play in 3 different tunings. Eb, drop C#, and drop B.
I play one bass for the first two tunings, just retuning the bottom string, then for the drop B stuff I just play a 5 or 6 string in standard tuning. Swapping basses and checking tuning can be done in seconds now I'm used to it.
...though I have forgotten to swap before and had to improvise