Unfortunately, it is not unusual for even the most reliable and decent of rental companies to have a change a rental schedule. The owners change their minds or neglect to inform the Agencies of changes - I think it is a real shame for a renter who thinks he/she has the apt. settled, and then has to re-group. Sometimes, however, one gets an even better apt. on the second round....this happened to us several years ago.
I love the general area you describe for this new location; and we will be staying in that neighborhood for the second time this year. You are near to the parks and to Evita Museum and cafes and restaurants and, and .....the zoo, the Palermo restaurants too.
As you do not give a specific address, I would suggest that you ask the rental agency about noise (this is always an issue in Buenos Aires) and learn if the apt. is facing the rear of the bldg. and on what floor, or if the windows are double pane. They will likely be honest, if it is a good company.
Enjoy your NEW neighborhood.
~MarnieWDC