I Have 6 Veneers Currently, I Need to Replace One. Will I Be Able to Just Repleace One Veneer?

You don't really need to replace all six veneers. If you only need to change one then you should just make sure they use the same shade and materials as the others. Be open with your dentist and express all your concerns, if you're not happy then get a second opinion. It's your smile and you should have it the way you want it to be.

Yes, but best to return to the original dentist who placed your veneers six years ago.  He should have a record of what color (shade) and type of porcelain.  If you're lucky he could even use the same ceramist.  Matching older dentistry can be a bit challenging, but not impossible.

If one tooth is moving than there is surely an underlying problem causing this.  An unstable bite?  Oral habits like smoking a pipe, biting on a pencil, etc.  Periodontal disease?  Suggest that you also speak with your dentist about getting a retainer or night guard bruxism appliance so that after he makes your new veneer it will keep that tooth from moving out of alignment again.

Yes you can just have one of the six veneers replaced.  With previous records of shading used by the lab and some good photos a good cosmetic lab should be able to match the new restoration to your old veneers.  You should have the reason your tooth keeps shifting evaluated and resolved with the new veneer.  Possibly you have an issue with how you swallow that keeps pushing the tooth forward.  Some sort of retainer should be considered to keep the tooth in place minimally.