So I'm making this architecture-themed cake for a customer and she had originally told me she wanted a chocolate cake with vanilla frosting for Saturday. I was planning on using white frosting sheets to draw a floor plan on, to use on the 13x9 cake. I have already bought all of my ingredients and had everything planned. Today, she sent me a message asking me to change the cake to carrot cake with cream cheese frosting and have it ready for FRIDAY (two days from now) instead of Saturday. First of all, should I be OK with her making all of these changes last minute? Second of all, if I do cream cheese frosting, will a vanilla frosting sheet over the top make the cream cheese frosting taste weird?? HELP!!

Do you have any kind of contract showing what her original order was? Is there any statement that NO changes can be made after (state date)?
I definately would inform her that cr ch icing is not possible in such hot weather or for the use of the theme she wants. Maybe suggest you could use it as a filling in the cake?
Or try Earlene Moore's cr ch b'cream. That would give a slight cr ch taste but still have the 'usability' of b'cream for decorating.
As for changing the p/up date: you could always inform them: sorry no can do, that you already have a full schedule for that day

I'm guessing there isn't a contract that addresses changes to orders.
Tell her there will be an additional charge for all the changes and that NO more changes can be allowed.

<rude word> like that happens in the software business, too.

I'd tell her that you'd already bought the ingredients for the contracted order, and if she wants to change it you'll have to charge her a fee to cover the new ingredients, your time to shop again, and a rush fee to move the date up. Chances are she'll go back to her original choice, but if she pays you for the changes at least you'll have that.