I have done it a few times in my life - never a problem. I've even had a couple occasions where I called UA to change my flight and had the agent inform me that a sub-$150 one-way segment was available and that I should just book that instead. (Happens on DEN-MCI and ORD-MCI quite often.)

I've never had the conversation with an airline employee about the rules, but I have with our corporate travel agent in the past. Her take was similar to what HomerJ heard: the go after people who chronically circumvent tariff rules with throwaway, back-to-back, or hidden-city ticketing.

Or, to be more specific, they go after travel agents who enable many people to do these things. That's why our corp agent always played it clean - especially the back-to-backs. The flier might not catch hell for doing it once, but the agent who lets 50 people do it once each will.

The Ireland situation from the post above seems like fair game. If you're concerned, call and cancel the existing ticket first. You'll have some credit in that PNR with a $250 fee to be paid if you choose to redeem it. Then buy the fresh new one-way ticket using a credit card. There isn't even a rules question here - you aren't required to use that PNR before buying other unrelated DL tickets. In fact, you aren't required to use it ever.

I suspect the OP is okay too but we don't have enough info to be sure.