Hello! I have an urgent question:

I booked a flight on United/CO to GVA and on the EWR to GVA leg, I got a message that there were no other "complimentary" seats available. I've never been in that predicament. I did not buy the economy plus seat which costs an extra $100. I tried looking for alternate flights but was unsuccessful. I called United this AM and the agent said the airline has blocked about 20 complimentary seats and that they would be released 24 hours prior to check-in and that the flight wasn't overbooked. I'm worried that when it's time for me to check-in, my only option will be to buy a more expensive seat. My flight already cost $1200 and I don't want to spend a penny more.

So, should I be worried that I may not get a regular economy seat? Should I cancel (I have 24 hours to change my mind) and look for something else? Is this not a big deal and just a tactic by United to sell economy plus seats (which the agent alluded to on the phone)? Should I just pay the $100 for the peace of mind of having a seat? Also, is this a normal occurrence? Thanks for your thoughts!