It's quite possible that they moved from a pension with a commutation option to a lower pension plus a lump sum as part of a Section 50 order. Your uncle should ask them to send him a copy of the communication that was issued at the time of the Section 50 order. One of the requirements for trustees seeking a Section 50 order is a 30 day consultation period, so there should be paperwork relating to the change. If they are not prepared to provide this information, your uncle should contact the Pensions Authority directly.
Having said this, I'm surprised that he received a statement in Nov 2017 showing a pension or lower pension with lump sum option if a change was implemented in 2014. Something doesn't quite add up. Send me a pm if you want to discuss further.