If you have a github account and linked to the smartapp in IDE, you only need to click on update from repo in the smartapp section. If you do not have a github account, you can edit the smartapp and copy/paste the new code over the old code and save/publish. Either method of updating will not cause you to lose your existing pistons.
there is no migration tool to move your pistons from core to webcore. I would suggest starting with the simple pistons first and see how webcore works. The installation guide and videos can can found on the webcore forum at https://community.webcore.co and their wiki page https://wiki.webcore.co/
A user account is required for their forum.
https://community.smartthings.com/t/webcore-forum-username-policy-verification/136055
You can find many example pistons on the webcore community forum and you can post any and all questions that you may have over there. The folks love to assist and enjoy helping with building pistons 