PI UPgragde from 7.11 to 7.3 Java Stack

Hi Experts,

My company is planning to upgrade from PI7.11 to PI7.3 Java Stack only.

1. As we are on dual stack and planning to move to Java Stack,  should we be doing Upgrade or New installation is required?
2. What all things I should be careful off?
3. Is there any checklist available which can help me to be on track?
4. We have IDOC, RFC Adapters heavily used in our interfaces, is there any advise from your experience?
5. We have  550+ interfaces in current system. Is there any time frame should you recommend?
6. We have almost 16 BPM interfaces, is it hard to rebuild the new BPM interfaces in PI 7.3?

Please feel free to give any advice if I missed any question,

This is going to be really critical project for us, really appreciate your help with this planning.

Regards

Ria