I am looking for a program, preferably command line or web based, that will convert .vcf
files.
Specifically, I am looking to convert
VCARD VERSION 2.1
To
VCARD VERSION 3.0
You can use convcard (free, Linux, CLI). It is part of msynctool, which is a CLI for OpenSync.
It converts a vCard version 2.1 in a vCard version 3.0 and vice versa. By default the source file format is autodetected, and is converted to the other format.