How will be the LTS support for Xubuntu after 3 years?

Hi :) I'm comfortable with Xubuntu 12.04 because I have an older computer, but I was reading about the LTS support:

• Ubuntu 12.04: Supported until april 2017.
• Xubuntu 12.04: Supported until april 2015.

I want a 5 years support, then my question is: Will be those 3 years support just for XFCE packages or for all packages? I want ask: After 3 years, will Xubuntu have updates for packages as Firefox, LibreOffice, kernel...?

If the answer is no (I suppose it), I could change the 'apt' repositories, from 'xubuntu' to 'ubuntu'. How do you see this solution?

The core aspects of Ubuntu get full support for the full period. That includes kernels, pre-installed applications. You should be fine with Firefox updates. It's the XFCE stuff that will start to lag behind.

The repositories for Xubuntu and Ubuntu are the same — it's just a different selection of packages. If you want full Xubuntu support indefinitely you either need to upgrade in a timely manner or maintain (or pay somebody else to maintain) all the packages you need indefinitely.