Hi guys. I'll try to help you install new os which is from the other carrier. Since I've had the same problem I was trying to figure it out a couple of times why my DM doesn't see the new OS if I followed the instructions, AppLoader neither did. I was even trying to install it using BBSAK but even this program couldn't see the new OS installed. So since the new OS 7.1 was annouced by Bell Mobility for my BB Curve 9360 I've decided that I won't give up

Finally I upgraded to OS 7.1

I did it after reading a lot of instructions how to do it and trying a lot of manners. On my notebook(windows 7) it didn't work using the instruction from this post. then I went to my friends house to see some football game and it worked out on his notebook(vista 64bit - I don't know if this is significant) but in other manner. He was never trying to update OS, so everything was untouched (vendor,etc...). Just had the DM installed. My way how I did it was much easier I think and more clarify because switching off the internet and connecting to DM was useless:
1. I downloaded the new OS 7.1
2. I deleted vendor.xml which is in c:/Program Files(86x)/Common Files/Reasearch In Motion/AppLoader/
3. I installed the OS 7.1
4. After installing the OS vendor.xml was created again. So I deleted it again.
5. I connected my phone using USB cable(DM is switched off)
6. I turned on Loader.exe which is in c:/Program Files(86x)/Common Files/Reasearch In Motion/AppLoader/
7. Loader detected that my phone is connected, I pushed "next" and the all aplication to install showed, including the upgrade to the new OS 7.1. Then I followed the instructions and succeed
Hope that this instruction will help you guys as it did for me. I think on my notebook it will work either but I didn't try it since I've already udpdated my BB. Try to uninstall the OS that you have already installed on your windows and DM first if you have been trying to delete vendor already and changing something. then install DM as a new one and try to follow the instruction