4.2.1 is mostly a bugfix release so upgrade is possible. But I recommend to make a test upgrade of your live store CLONE and try to upgrade this clone first.
check this
http://forum.x-cart.com/showthread.php?t=46853
PS
To fix this particular bug only
replace in your skin1/customer/main/subcategories.tpl
Code:
{math equation="x+15" assign="standoff" x=$current_category.image_x}
with
Code:
{math equation="x+15" assign="standoff" x=$current_category.image_x|default:0}