Making two printed parts fit after switching filament brands

Two parts that fitted beautifully with one PLA brand are now frustratingly tight using another. Same CAD dimensions and printer, but the new filament produces slightly different wall measurements. I can force the joint together, though that's not a solution for a repeatable design. Do you calibrate flow for each spool, build in more clearance for interchangeable filaments, or print a small tolerance coupon first? It's a snap-fit feature, so a fraction of a millimeter matters.