Just about to start a RTW - I had planned to only use 15 segments and held back from using the 16th in case I had an 'involuntary change' forced on me (say having one of my direct flights cancelled and having to take an indirect route)

I would like to use the 16th segment but am I taking too much of a risk ? Should I hold it back in case or will it be OW's problem if I get a flight cancelled and have to exceed the 16 limit ?
Thanks

Over the years each oneworld airline has always made any reasonable changes needed as a result of involuntary changes. Bottom line: no need to worry here..

You have the following risk: If you later want to do a date change and no OW airline is flying that sector any more then you need to reroute and potentially don't have enough sectors left. If the airline forces a change on you they find a solution without affecting the sector count of the original ticket. That is my experience.

If all 16 sectors on a ticket are used and a flight is changed which requires an extra sector to fix the issue, that is the airlines problem to fix.
If there is an involuntary change for whatever reason, the airline can break whatever rule it wants, including the 16 sector e-ticket limit.
There is no reason to 'hold back' on a 16th sectors "in case" of a change.

In 2008, just as the paper ticket phase out deadline was about to hit, I booked a 16 sector LONE4. CBR-SYD-JFK-BWI-ORD,SEA-LHR-MAD-FCO-LHR-BKK-SIN-KIX-HND,NRT-MEL-CBR
After ticket issue, QF caned the NRT-MEL flight and I was rerouted NRT-SYD-MEL, making it 17 sectors. It was reissued as a paper ticket.
Later, during the trip, they starting having political problems in BKK, including rioting with multiple airlines and governments issuing travel waivers and warnings. While I was in MAD, I had the ticket changed to bypass the BKK stop and the remaining sectors (MAD-FCO-LHR-SIN-KIX-HND,NRT-SYD-MEL-CBR) were reissued as an e-ticket (and I had to return the remaining paper coupons to the travel agent upon return, though BA still insisted on using the now invalid paper ticket for FCO-LHR).

More recently, some people have been given 17 and 18 sector trips due to invol reroutes (such as having to reroute on some QR middle east flights). The airlines have a number of ways to sort that out (such as splitting the trip into 2 tickets and linking them, or leaving notes in the PNR to rebook once some sectors at the start of the trip have been used).

Appreciate all the responses and will now use my 16th segment!!

Flown: CAI-LHR-DOH-CDG-DOH-LAX-ORD-SJU-JFK-SFO-ORD-HKG-MLE-HKG-DPS

This sunday: DPS-HKG canceled

Rebooked on KE(!) to become DPS-ICN-HKG-TLV. No issues and ticket still intact under 1 PNR.

What was your original routing after DPS?

DPS-HKG-TLV seems logical. In that case you got one extra segment, and 2.4K more miles.

However , I hope you have somewhere to credit all 4.5 K miles from KE, as your profile only lists 2 OW FFP.