Basically as above. 

An old silverstone HTPC case with front USB 2.0. 

Can I buy USB 3.0 header cables and remove the old ports and perhaps glue the replacement ones in? 

The spacing between the USB ports is quite tight and doesn't seem to match the spacing of the off the shelf PC front port upgrade kits.

EDIT, not that such kits would work as the HTPC doesn't have a user replaceable accessory port unit, its simply USB ports recessed into the face of the case.