Im shopping for a motor for my boat and wondering whats the difference between a standard 9.9(4 stroke Suzuki) and a high thrust 9.9(4 stroke Suzuki).  If i buy a standard, could i convert it into a high thrust sometime in the future if i wanted??

High thrust has a bigger gear case.  They are sometimes recommended when a boat carries a heavier than normal load.   Not sure if converting a std to high thrust could be done.  It would be expensive to do I would think.  

The high thrust motors are the ones used on sailboats for docking. Very strong but very slow.

if its your main motor I would go for the standard motor. the High thrust is used for a kicker,  sailboats and, poontoon boats.

HT outboards are for pushing alot of weight. They ussially have lower gears & lower pitch bigger props that let them push more weight. They  are not fast.