Helms of opposite alignment don’t instantly change your alignment. They still autocurse, but they instead constantly drain your alignment record, until it hits some negative value (-1? -100?) which triggers your alignment to change. When this happens, your alignment record is reset to 0, and no longer drains as long as your alignment remains something other than your original one.

If you remove the helm, it works similarly, rather than immediately restoring your initial alignment: with no helm of opposite alignment on and with a non-starting alignment, your alignment record constantly drains. When it hits that same negative threshold, your alignment changes back to its starting value and alignment record is reset to 0.