Losing low is a rare failure (usually the last). To get low, the 2nd/high driving ratchet has to be disengaged. This is done by having the tips of the clutch engage a raised area on the back sides of the high gear pawls withdrawing them so they can't engage. The drive now happens via the low gear pawls on the left side of the hub.
When the clutch is a bit worn so it doesn't withdraw the pawls enough to disengage them, low and second would be the same. this is a simple, inexpensive repair involving replacing the clutch and/or (I do both) the high gear pawls.
If by losing the low, you mean that there's no drive in low (slips, no power) then the problem is that the low gear pawls are worn, dirty or gummy or that the low gear ratchet ring is worn or chipped. Then when the high gear pawls are disengaged transferring power to the low side, they slip. Usually this problem is more related to dirt or dried lube than to actual wear. Also an easy fix.
Either way, you're looking at a few easy to replace parts plus a clean and oil job, so there shouldn't be any need to replace any but the involved parts (if any).
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