Override CRM Online Ribbon Button Functionality

Hi

I have to implement one validation on click of "Close As Won" button on opportunity form. I am thinking of following two options:

1. Write a pre-operation or pre-validation plugin on "Win" Message. Do the Validation in Plugin if it fails, then throw exception.

2. Change the Command of Ribbon Button and use your own Javascript function. Validate in JavaScript, if validation fails throw error message else call OOB Command function of ribbon button.

I am worried that with option 2 , if CRM Online gets updated then it might override the changes made by me. Please suggest.