A regular 26" rim is 559 diameter, S7 is 571 mm. Difference of 12 mm, but only 6 mm radius. If the spoke threads past the end of the nipple, grind it off. You then use the same spokes. That's what the guys at the bike shop do where I've taken some oddball wheels I've laced up. Take the hand held grinder to the threads sticking up past the nipple.
If the wheel now is laced 3 cross, you may also be able to lace it 4 cross to make up the difference and use the same spokes. To determine the exact spoke length will depend on the size of the hub and the size of the rim. I use a calculator to get the size. If my spokes are 3 or 4 mm different, it still works out most times.
Spoke length calculator to determine the correct spoke length for your bike. An illustrated guide: Spoking your bicycle wheel in 6 steps!
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