The instinct when a trip crosses multiple climates is to pack for each one separately, which is how a carry-on turns into a checked bag. The actual solution is a layering system that reshuffles for temperature rather than swapping entire outfits.
The three-layer base
A base layer (moisture-wicking t-shirts you already packed for the warm leg), a mid layer (one fleece or light sweater), and a shell layer (one packable rain/wind jacket) cover almost every temperature between roughly 35°F and 85°F when combined in different orders.
What actually needs duplicating
Socks and underwear, full stop — everything else can be re-worn or layered differently. If the trip includes genuinely cold weather (below freezing), that's the one case to add a single dedicated warm item, like merino wool base layers, rather than trying to force summer clothes to do double duty.
Footwear is the hard constraint
One pair of broken-in walking shoes and one pair of sandals or lightweight flats covers the vast majority of four-season trips. Resist packing boots "just in case" for a two-day cold stretch — rent or buy cheap gear locally at your destination instead if genuinely necessary.
Takeaway: Layer for temperature swings instead of packing separate wardrobes, and only add one truly cold-specific item if the trip includes freezing temperatures.