Five years ago we had our trip all planned and just a couple weeks before the trip we thought "what are we doing driving in a car for 23 hours with a 3 month old? So we ended up finding tickets for a steal. The only problem was we could not get seat assignments for one flight segment. Not even the day before travel, or when checking in at the first airport or upon arrival at the airport where we had no seat assingments. We had to wait until everyone boarded ( I was very nervous about this, having 3 kids along) but they finally called our name and bummer they only had room in first class would that be ok? YEAH!!
Also we are traveling in 3 weeks and I just purchased different tickets for our travel because prices became so low, the new flights are at much better times and from a closer airport, so I couldn't resist. At first I could not get all seats together, but in the 2 weeks since we bought them I have been watching the seat availability on the flights and called to rebook our seats so now we are together.
Good luck with whatever you decide.