Adding KPI product category Upgrade/Downgrade index.html.md.erb

Why is this change being made?

This will help the company better understand the conversion rate of customers from tier to tier and help us make recommendations to enhance our tiers for greater adoption by existing customers and prospects.

We are documenting it in the handbook for transparency.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what