Your milage is nly set to zero at the end of 12 months so if you get silver you can carry on up to gold or whatever during the same 12 month period then it is re-set to zero.

Star Alliance is far more generous in miles (except SG).
CX is pretty mean - they dont give any bonus miles or upgrade certs for being top tier. They do give free upgrades if they are full and guaranteed seat with 24 hours notice - that is the best bit. oh, and a free bottle of water if you are in Y !!