Have Bitcoin's consensus rules ever been changed?

[wingding]

if so, how often, and on what?

[amaclin]

if so, how often, and on what?

every time on soft-fork activated

[achow101]

Certainly. The consensus rules change with any fork, be it hard or soft. All planned forks to date have been soft forks. To see what these are, look at the list of BIPs: https://github.com/bitcoin/bips/blob/master/README.mediawiki. Anything listed as "Consensus (Soft fork)" and has a status of "Final" are all of the BIPs for soft forks that have already been activated.