Thanks everyone for the responses. I really appreciate it. I was on an award ticket and didn't use SWU. The ticket I purchased for my daughter was 10% of the Y/M/B fare but I didn't realize that I could pay the difference in order to keep the U/G. That's something that I would consider if I encounter this situation again.

My main concern was that the supervisor at the gate clearly stated that I was being downgraded due to having an infant in arms- not because I was on an award ticket or because I paid 10% of a Y/M/B fare. Upgrades are *the* perk that I enjoy as a Platinum so I wouldn't want to fly with DL if I couldn't benefit from that when flying with an infant in arms. It appears some of you have benefited from U/Gs internationally and domestically so it shouldn't be an issue. Internationally I would just need to pay the 10% difference between F/J and Y/M/B. I have a couple of domestic DL flights this week with my infant so we'll see if I have better luck.

Thanks for your responses!