So after doing a couple dozen tests, it does in fact seem like svchax works BETTER than memchunkhax on FIRM versions under 10.4 as well.
Took me dozens of tries to get it to work using the CPU race, but worked every time with GPU - tested with o3DS.
IF YOU ARE DOWNGRADING FROM 10.3 OR BELOW (NOT 10.4 - 10.6!!), PLEASE TRY THIS VERSION OF THE UPDATER AND REPORT YOUR RESULTS AND LET ME KNOW IF YOU ARE ON o3DS/n3DS!
DO NOT TEST THIS ON ANY 3DS WITH CFW/A9LH! Userland 3DS ONLY using HBL!
Only report if you got HAX SUCCESS + downgrade starting, or a freeze. Do NOT report partials/downgrade errors themselves.
Particularly, let me know if it worked better than the updater posted in the OP.