@Abhishek Raj - start with reviewing the documentation for new features, and Upgrade Compatibility checklists:

This will be the most comprehensive collection of suggestions, things to know, and 'gotcha's' to be aware of.  

Beyond reading the docs and seeing what is applicable to your environment, do a practice upgrade of your application and TEST, TEST, TEST :)  

Lastly, make sure you review the supported platform list for 2018.1 and 2015.2:

2018.1: https://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY...

2015.2: https://cedocs.intersystems.com/ens20152/csp/docbook/DocBook.UI.Page.cls...

Hope that helps!

@Alexey Maslov  - check out https://openexchange.intersystems.com/package/isc-codetidy .... this package includes automatic expansion of ObjectScript commands.  Even if you don't want to use the whole package, you can find the sample code to do expansion.

@Evan Gabhart  - can you please advise as to the specific class where the expansion code can be found?