This gives you the number of set bits, not the next larger number with the same number of set bits.

Filter larger, zip Popcount, filter with same popcount, min.

If you need the full implementation spelled out I can do that when I get in from my commute home.

Step 1 is computing a set of ~2^64 things?

Lol just gotten in and looked at this at a desktop I thought we were filtering a list. My bad. Funny though.

Interesting task I guess really you want to find the first (from right) and move the first set bit left to fill it.

You could keep incrementing from the current value until the population count matches again, but in the worst case - probably from 2^62 to 2^63 with a single set bit or wrapping around from 2^63 to 2^0 - that would still take pretty much 2^64 steps.