Can I upgrade a 133c to AP for just a couple of clients just by upgrading key? Or do I have to install additional components in it. Having a major interference problem with a client who is lower down the street and being hit by a big towers RF, but his neighbor up the street has a great signal with little to no interference so I was thinking instead of installing a filter on his CPE well just point the opposite way up the street to my good signal CPE...???

You can, but you do have to pay full price for the upgrade - $45USD I believe.