If you registered to vote a certain party, do you have to re-register in order to vote for another party?

Yes, you will need to re-register. You can do so with your county election official, usually these forms are available online.

One advantage to registering with a specific party is that you can vote in some primary elections, whereas you cannot if you register as an independent.

You must be registered with a certain party in order to vote in the primaries.

So if you want to vote for Hillary or Obama, you have to be a registered democrat. Of if you want to jump on the Ron Paul band wagon and help get him nominated you have to be a republican.

The deadline to switch your voter status to a new party is past.

Seeing how I'm still a republican I'll probably vote Ron Paul in the Colorado primary, even though I may end up voting for Obama come Nov.