keyless entry remote question

Curious... If you get a used car that has keyless entry and it is missing the remote (key fob).. You buy a replacement remote and program it... If someone else has the original ..would it still unlock the vehicle if a
replacement has been newly programmed?
Been reading that most, if not all, remotes have to be programmed at the same time.. So if you program a new/replacement remote...the original shouldn't work? (unless programmed at same time) Is this correct?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

From http://auto.howstuffworks.com/remote-entry2.htm , the old remote should not work with the system reprogrammed. Of course, someone will reply who knows someone who knew someone who had a friend and it did work.
C R
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.