Creating a new and Customizing ODK Collect App

1. What is the issue? Please be detailed.
I have 2 questions. Initially we were using a customized application which was talking with odk Aggregate server. So my questions are as followed:

  1. Can I use the previous android app which was getting connected with the ODK Aggregate Server DB, will the same app work for ODK Central?

  2. Can I create a new apk and how can I create a custom apk which will work with the ODK Collect

2. What steps can we take to reproduce this issue?
I tried to connect the old apk with the new ODK Collect server but it didn't work

3. What have you tried to fix the issue?

4. Upload any forms or screenshots you can share publicly below.