What happens if I choose change now? Will I lose whats remaining of my old plan?

No you will not. You will keep what you have and the difference of the new plan and your previous plan will be added to your remaining balance

Managing my plan
What happens if I renew my plan early?
What is the plan recommendation feature?
How can I cancel my plan with Virgin Mobile?
Where can I see my plan?
When will my plan be renewed?
How do i get the best prices with Virgin Mobile?
What happens if I choose (change on renewal)?